A question about the number 541456












0














I found that the concatenation in base 10 of $2^{541456}-1$ and $2^{541455}-1$, gives a probable prime.
I also found about number $541456$ that:



$$5cdot(5^2+4^2+1+4^2+5^2+6^2)=(5^3+4^3+1+4^3+5^3+6^3)$$



the equation $acdot(2cdot a^2+2cdot b^2+c^2+d^2)=(2cdot a^3+2cdot b^3+c^3+d^3)$ besides $a=5, b=4, c=6, d=1$ has other non trivial solutions with a,b,c,d positive integers?










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  • Do you mean different values? If not, take $a=b=c=d=1$.
    – AugSB
    Jan 4 at 20:52










  • As a rough estimate based on the prime number theorem there ought to be about three $k$-digit numbers such that the base-ten concatenation of $2^{n+1}-1$ and $2^n-1$ is prime, for each $k$.
    – Henning Makholm
    Jan 4 at 20:58






  • 1




    A suggestion: replace the two $d$'s in the equation you ask about with a $d^2$ on the left and a $d^3$ on the right.
    – Barry Cipra
    Jan 4 at 21:08












  • Why not make this homogeneous on each side since $1=1^2=1^3$? This is equivalent to @BarryCipra's comment.
    – Mark Bennet
    Jan 4 at 21:13










  • @Barry Cipra now i replaced as you suggested
    – user631751
    Jan 4 at 21:33
















0














I found that the concatenation in base 10 of $2^{541456}-1$ and $2^{541455}-1$, gives a probable prime.
I also found about number $541456$ that:



$$5cdot(5^2+4^2+1+4^2+5^2+6^2)=(5^3+4^3+1+4^3+5^3+6^3)$$



the equation $acdot(2cdot a^2+2cdot b^2+c^2+d^2)=(2cdot a^3+2cdot b^3+c^3+d^3)$ besides $a=5, b=4, c=6, d=1$ has other non trivial solutions with a,b,c,d positive integers?










share|cite|improve this question









New contributor




user631751 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.




















  • Do you mean different values? If not, take $a=b=c=d=1$.
    – AugSB
    Jan 4 at 20:52










  • As a rough estimate based on the prime number theorem there ought to be about three $k$-digit numbers such that the base-ten concatenation of $2^{n+1}-1$ and $2^n-1$ is prime, for each $k$.
    – Henning Makholm
    Jan 4 at 20:58






  • 1




    A suggestion: replace the two $d$'s in the equation you ask about with a $d^2$ on the left and a $d^3$ on the right.
    – Barry Cipra
    Jan 4 at 21:08












  • Why not make this homogeneous on each side since $1=1^2=1^3$? This is equivalent to @BarryCipra's comment.
    – Mark Bennet
    Jan 4 at 21:13










  • @Barry Cipra now i replaced as you suggested
    – user631751
    Jan 4 at 21:33














0












0








0







I found that the concatenation in base 10 of $2^{541456}-1$ and $2^{541455}-1$, gives a probable prime.
I also found about number $541456$ that:



$$5cdot(5^2+4^2+1+4^2+5^2+6^2)=(5^3+4^3+1+4^3+5^3+6^3)$$



the equation $acdot(2cdot a^2+2cdot b^2+c^2+d^2)=(2cdot a^3+2cdot b^3+c^3+d^3)$ besides $a=5, b=4, c=6, d=1$ has other non trivial solutions with a,b,c,d positive integers?










share|cite|improve this question









New contributor




user631751 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.











I found that the concatenation in base 10 of $2^{541456}-1$ and $2^{541455}-1$, gives a probable prime.
I also found about number $541456$ that:



$$5cdot(5^2+4^2+1+4^2+5^2+6^2)=(5^3+4^3+1+4^3+5^3+6^3)$$



the equation $acdot(2cdot a^2+2cdot b^2+c^2+d^2)=(2cdot a^3+2cdot b^3+c^3+d^3)$ besides $a=5, b=4, c=6, d=1$ has other non trivial solutions with a,b,c,d positive integers?







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user631751 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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share|cite|improve this question









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edited Jan 4 at 21:20







user631751













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asked Jan 4 at 20:39









user631751user631751

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11




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user631751 is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






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Check out our Code of Conduct.












  • Do you mean different values? If not, take $a=b=c=d=1$.
    – AugSB
    Jan 4 at 20:52










  • As a rough estimate based on the prime number theorem there ought to be about three $k$-digit numbers such that the base-ten concatenation of $2^{n+1}-1$ and $2^n-1$ is prime, for each $k$.
    – Henning Makholm
    Jan 4 at 20:58






  • 1




    A suggestion: replace the two $d$'s in the equation you ask about with a $d^2$ on the left and a $d^3$ on the right.
    – Barry Cipra
    Jan 4 at 21:08












  • Why not make this homogeneous on each side since $1=1^2=1^3$? This is equivalent to @BarryCipra's comment.
    – Mark Bennet
    Jan 4 at 21:13










  • @Barry Cipra now i replaced as you suggested
    – user631751
    Jan 4 at 21:33


















  • Do you mean different values? If not, take $a=b=c=d=1$.
    – AugSB
    Jan 4 at 20:52










  • As a rough estimate based on the prime number theorem there ought to be about three $k$-digit numbers such that the base-ten concatenation of $2^{n+1}-1$ and $2^n-1$ is prime, for each $k$.
    – Henning Makholm
    Jan 4 at 20:58






  • 1




    A suggestion: replace the two $d$'s in the equation you ask about with a $d^2$ on the left and a $d^3$ on the right.
    – Barry Cipra
    Jan 4 at 21:08












  • Why not make this homogeneous on each side since $1=1^2=1^3$? This is equivalent to @BarryCipra's comment.
    – Mark Bennet
    Jan 4 at 21:13










  • @Barry Cipra now i replaced as you suggested
    – user631751
    Jan 4 at 21:33
















Do you mean different values? If not, take $a=b=c=d=1$.
– AugSB
Jan 4 at 20:52




Do you mean different values? If not, take $a=b=c=d=1$.
– AugSB
Jan 4 at 20:52












As a rough estimate based on the prime number theorem there ought to be about three $k$-digit numbers such that the base-ten concatenation of $2^{n+1}-1$ and $2^n-1$ is prime, for each $k$.
– Henning Makholm
Jan 4 at 20:58




As a rough estimate based on the prime number theorem there ought to be about three $k$-digit numbers such that the base-ten concatenation of $2^{n+1}-1$ and $2^n-1$ is prime, for each $k$.
– Henning Makholm
Jan 4 at 20:58




1




1




A suggestion: replace the two $d$'s in the equation you ask about with a $d^2$ on the left and a $d^3$ on the right.
– Barry Cipra
Jan 4 at 21:08






A suggestion: replace the two $d$'s in the equation you ask about with a $d^2$ on the left and a $d^3$ on the right.
– Barry Cipra
Jan 4 at 21:08














Why not make this homogeneous on each side since $1=1^2=1^3$? This is equivalent to @BarryCipra's comment.
– Mark Bennet
Jan 4 at 21:13




Why not make this homogeneous on each side since $1=1^2=1^3$? This is equivalent to @BarryCipra's comment.
– Mark Bennet
Jan 4 at 21:13












@Barry Cipra now i replaced as you suggested
– user631751
Jan 4 at 21:33




@Barry Cipra now i replaced as you suggested
– user631751
Jan 4 at 21:33










3 Answers
3






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2














Your equation $a*(2*a^2+2*b^2+c^2+d)=(2*a^3+2*b^3+c^3+d)$ is equivalent to
$$2ab^2+ac^2+ad=2b^3+c^3+d$$
I find the below solutions assuming all of the variables are in the range $1$ to $9$. I just did a search.



enter image description here






share|cite|improve this answer























  • Mllikan besides a=1, b=1, c=1, d=1, the only solution with d=1 seems to be a=5,b=4, c=6, d=1?
    – user631751
    Jan 4 at 22:05












  • That is correct if you want $a,b,c,d$ to be nonzero digits. If you don't restrict the values to $9$ or below Eric Towers has more cases.
    – Ross Millikan
    Jan 4 at 22:12












  • can you proof that?
    – user631751
    Jan 4 at 22:14












  • I did a complete search in the space I described. That is a fine proof (assuming it is done correctly). There are only $9^4=6561$ cases to check
    – Ross Millikan
    Jan 4 at 22:16





















2














This section of this Answer is responsive to the version of the Question which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d^2) = (2 a^3 + 2 b^3 + c^3 + d^3)$.



Notice that $c$ and $d$ are interchangeable, so we may impose $c leq d$.



One way to look at this is $b,c,d geq 1$ and $a =frac{2 b^3 + c^3 + d^3}{2 b^2 + c^2 + d^2}$, when that is an integer. This leads to the question, does that ever happen? Yes. Notice if $b = c = d$, this is $a = (4b^3)/(4 b^2) = b$, so $a = b = c = d$ is a solution for any choice of $a$.



begin{align*}
&(a,b,c,d) & text{giving} \
&(4,4,4,4) & 384 \
&(5,5,5,5) & 750 \
end{align*}



Cylindrical algebraic decomposition with variable order $c, d, a, b$, gives five cylindrical components. Exhibiting an integer point in a component or showing that a component has no integer points is not easy.





  • $c geq 1$, $d geq 1$, $a = b = frac{c^3 + d^3}{c^2 + d^2}$, when this is an integer. The fraction is always an integer when $c = d$ and is sometimes an integer otherwise. Examples: $(1,1,1,1)$, $(9,9,5,10)$


  • $d geq c geq 1$, $a > frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the least real root of $2x^3 -2ax^2 -a(c^2 +d^2) +c^3 + d^3$. Examples: $(9,10,5,5)$, $(17,18,2,14)$, and $(251,255,51,251)$.


  • $d geq c geq 1$, $a$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when those roots are integers. (In progress: determining whether those roots are ever integers.)


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(4,2,3,5)$ and $(5,2,2,6)$.


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the largest root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(5,4,1,6)$ and $(17,15,5,20)$.




The below is responsive to the first version of the Question, which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d) = (2 a^3 + 2 b^3 + c^3 + d)$.



Well, $a = b = c = d = 1$ is pretty obvious, giving $6$ on both sides.



One infinite family is $a=b=c = 1$, $d$ any positive integer, giving $d+5$. There don't seem to be more with $a = 1$, but I don't know how to prove that.



Here are eight more.
begin{align*}
&(a,b,c,d) & text{giving} \
&(2, 1, 7, 243) & 604 \
&(2, 1, 100, 979998) & 1980016 \
&(2, 2, 8, 384) & 928 \
&(2, 2, 68, 305184) & 619648 \
&(2, 3, 19, 6155) & 13084 \
&(2, 41, 1, 131117) & 268976 \
&(2, 41, 26, 147342) & 302776 \
&(2, 41, 27, 149343) & 306884
end{align*}



Cylindrical algebraic decomposition, using the variable ordering $b,c,a,d$ of your equation gives three solution families:





  • $a = b = c = 1$ and $d geq 1$, e.g., $(1,1,1,1)$ and $(1,1,1,108)$.


  • $b = 1$, $c geq 2$, $1 < a < frac{c^3+2}{c^2+2}$, $d = frac{c^3 - a c^2 - 2a + 2}{a-1}$, when $d$ is an integer, e.g., $(2,1,3,7)$ and $(109,1,126,2497)$.


  • $b geq 2$, $c geq 1$, $1 < a < frac{c^3+2b^3}{c^2+2b^2}$, $d = frac{c^4 - a c^2 + 2 b^3-2ab^2}{a-1}$, when $d$ is an integer, e.g., $(5,2,6,3)$ and $(3,3,100,485,000)$.


(The variable ordering $a,b,c,d$ gives more families, some of which appear to be empty, and most of which require bounds on $b$ and $c$ that are somewhat complicated algebraic functions of the prior variables. The previous version(s) of this answer used that ordering, but the new ordering is used to aid readability.)






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  • what if $aneq bneq cneq d$?
    – Enzo Creti
    Jan 5 at 19:12










  • they should be infinite
    – Enzo Creti
    Jan 5 at 20:37












  • no no ok thank you!
    – Enzo Creti
    Jan 5 at 20:40










  • @EnzoCreti : Even stronger: $|{a,b,c,d}| = 4$. $(4,2,3,5)$.
    – Eric Towers
    Jan 6 at 7:04



















0














An obvious solution lies where a,b,c,d all equal 1 as:
$$1cdot(2cdot1^2+2cdot1^2+1^2+1) = (2cdot1^3+2cdot1^3+1^3+1)$$






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  • this seems a trivial solution
    – user631751
    Jan 4 at 20:58











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3 Answers
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3 Answers
3






active

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active

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2














Your equation $a*(2*a^2+2*b^2+c^2+d)=(2*a^3+2*b^3+c^3+d)$ is equivalent to
$$2ab^2+ac^2+ad=2b^3+c^3+d$$
I find the below solutions assuming all of the variables are in the range $1$ to $9$. I just did a search.



enter image description here






share|cite|improve this answer























  • Mllikan besides a=1, b=1, c=1, d=1, the only solution with d=1 seems to be a=5,b=4, c=6, d=1?
    – user631751
    Jan 4 at 22:05












  • That is correct if you want $a,b,c,d$ to be nonzero digits. If you don't restrict the values to $9$ or below Eric Towers has more cases.
    – Ross Millikan
    Jan 4 at 22:12












  • can you proof that?
    – user631751
    Jan 4 at 22:14












  • I did a complete search in the space I described. That is a fine proof (assuming it is done correctly). There are only $9^4=6561$ cases to check
    – Ross Millikan
    Jan 4 at 22:16


















2














Your equation $a*(2*a^2+2*b^2+c^2+d)=(2*a^3+2*b^3+c^3+d)$ is equivalent to
$$2ab^2+ac^2+ad=2b^3+c^3+d$$
I find the below solutions assuming all of the variables are in the range $1$ to $9$. I just did a search.



enter image description here






share|cite|improve this answer























  • Mllikan besides a=1, b=1, c=1, d=1, the only solution with d=1 seems to be a=5,b=4, c=6, d=1?
    – user631751
    Jan 4 at 22:05












  • That is correct if you want $a,b,c,d$ to be nonzero digits. If you don't restrict the values to $9$ or below Eric Towers has more cases.
    – Ross Millikan
    Jan 4 at 22:12












  • can you proof that?
    – user631751
    Jan 4 at 22:14












  • I did a complete search in the space I described. That is a fine proof (assuming it is done correctly). There are only $9^4=6561$ cases to check
    – Ross Millikan
    Jan 4 at 22:16
















2












2








2






Your equation $a*(2*a^2+2*b^2+c^2+d)=(2*a^3+2*b^3+c^3+d)$ is equivalent to
$$2ab^2+ac^2+ad=2b^3+c^3+d$$
I find the below solutions assuming all of the variables are in the range $1$ to $9$. I just did a search.



enter image description here






share|cite|improve this answer














Your equation $a*(2*a^2+2*b^2+c^2+d)=(2*a^3+2*b^3+c^3+d)$ is equivalent to
$$2ab^2+ac^2+ad=2b^3+c^3+d$$
I find the below solutions assuming all of the variables are in the range $1$ to $9$. I just did a search.



enter image description here







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Jan 4 at 21:15

























answered Jan 4 at 20:59









Ross MillikanRoss Millikan

292k23197371




292k23197371












  • Mllikan besides a=1, b=1, c=1, d=1, the only solution with d=1 seems to be a=5,b=4, c=6, d=1?
    – user631751
    Jan 4 at 22:05












  • That is correct if you want $a,b,c,d$ to be nonzero digits. If you don't restrict the values to $9$ or below Eric Towers has more cases.
    – Ross Millikan
    Jan 4 at 22:12












  • can you proof that?
    – user631751
    Jan 4 at 22:14












  • I did a complete search in the space I described. That is a fine proof (assuming it is done correctly). There are only $9^4=6561$ cases to check
    – Ross Millikan
    Jan 4 at 22:16




















  • Mllikan besides a=1, b=1, c=1, d=1, the only solution with d=1 seems to be a=5,b=4, c=6, d=1?
    – user631751
    Jan 4 at 22:05












  • That is correct if you want $a,b,c,d$ to be nonzero digits. If you don't restrict the values to $9$ or below Eric Towers has more cases.
    – Ross Millikan
    Jan 4 at 22:12












  • can you proof that?
    – user631751
    Jan 4 at 22:14












  • I did a complete search in the space I described. That is a fine proof (assuming it is done correctly). There are only $9^4=6561$ cases to check
    – Ross Millikan
    Jan 4 at 22:16


















Mllikan besides a=1, b=1, c=1, d=1, the only solution with d=1 seems to be a=5,b=4, c=6, d=1?
– user631751
Jan 4 at 22:05






Mllikan besides a=1, b=1, c=1, d=1, the only solution with d=1 seems to be a=5,b=4, c=6, d=1?
– user631751
Jan 4 at 22:05














That is correct if you want $a,b,c,d$ to be nonzero digits. If you don't restrict the values to $9$ or below Eric Towers has more cases.
– Ross Millikan
Jan 4 at 22:12






That is correct if you want $a,b,c,d$ to be nonzero digits. If you don't restrict the values to $9$ or below Eric Towers has more cases.
– Ross Millikan
Jan 4 at 22:12














can you proof that?
– user631751
Jan 4 at 22:14






can you proof that?
– user631751
Jan 4 at 22:14














I did a complete search in the space I described. That is a fine proof (assuming it is done correctly). There are only $9^4=6561$ cases to check
– Ross Millikan
Jan 4 at 22:16






I did a complete search in the space I described. That is a fine proof (assuming it is done correctly). There are only $9^4=6561$ cases to check
– Ross Millikan
Jan 4 at 22:16













2














This section of this Answer is responsive to the version of the Question which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d^2) = (2 a^3 + 2 b^3 + c^3 + d^3)$.



Notice that $c$ and $d$ are interchangeable, so we may impose $c leq d$.



One way to look at this is $b,c,d geq 1$ and $a =frac{2 b^3 + c^3 + d^3}{2 b^2 + c^2 + d^2}$, when that is an integer. This leads to the question, does that ever happen? Yes. Notice if $b = c = d$, this is $a = (4b^3)/(4 b^2) = b$, so $a = b = c = d$ is a solution for any choice of $a$.



begin{align*}
&(a,b,c,d) & text{giving} \
&(4,4,4,4) & 384 \
&(5,5,5,5) & 750 \
end{align*}



Cylindrical algebraic decomposition with variable order $c, d, a, b$, gives five cylindrical components. Exhibiting an integer point in a component or showing that a component has no integer points is not easy.





  • $c geq 1$, $d geq 1$, $a = b = frac{c^3 + d^3}{c^2 + d^2}$, when this is an integer. The fraction is always an integer when $c = d$ and is sometimes an integer otherwise. Examples: $(1,1,1,1)$, $(9,9,5,10)$


  • $d geq c geq 1$, $a > frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the least real root of $2x^3 -2ax^2 -a(c^2 +d^2) +c^3 + d^3$. Examples: $(9,10,5,5)$, $(17,18,2,14)$, and $(251,255,51,251)$.


  • $d geq c geq 1$, $a$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when those roots are integers. (In progress: determining whether those roots are ever integers.)


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(4,2,3,5)$ and $(5,2,2,6)$.


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the largest root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(5,4,1,6)$ and $(17,15,5,20)$.




The below is responsive to the first version of the Question, which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d) = (2 a^3 + 2 b^3 + c^3 + d)$.



Well, $a = b = c = d = 1$ is pretty obvious, giving $6$ on both sides.



One infinite family is $a=b=c = 1$, $d$ any positive integer, giving $d+5$. There don't seem to be more with $a = 1$, but I don't know how to prove that.



Here are eight more.
begin{align*}
&(a,b,c,d) & text{giving} \
&(2, 1, 7, 243) & 604 \
&(2, 1, 100, 979998) & 1980016 \
&(2, 2, 8, 384) & 928 \
&(2, 2, 68, 305184) & 619648 \
&(2, 3, 19, 6155) & 13084 \
&(2, 41, 1, 131117) & 268976 \
&(2, 41, 26, 147342) & 302776 \
&(2, 41, 27, 149343) & 306884
end{align*}



Cylindrical algebraic decomposition, using the variable ordering $b,c,a,d$ of your equation gives three solution families:





  • $a = b = c = 1$ and $d geq 1$, e.g., $(1,1,1,1)$ and $(1,1,1,108)$.


  • $b = 1$, $c geq 2$, $1 < a < frac{c^3+2}{c^2+2}$, $d = frac{c^3 - a c^2 - 2a + 2}{a-1}$, when $d$ is an integer, e.g., $(2,1,3,7)$ and $(109,1,126,2497)$.


  • $b geq 2$, $c geq 1$, $1 < a < frac{c^3+2b^3}{c^2+2b^2}$, $d = frac{c^4 - a c^2 + 2 b^3-2ab^2}{a-1}$, when $d$ is an integer, e.g., $(5,2,6,3)$ and $(3,3,100,485,000)$.


(The variable ordering $a,b,c,d$ gives more families, some of which appear to be empty, and most of which require bounds on $b$ and $c$ that are somewhat complicated algebraic functions of the prior variables. The previous version(s) of this answer used that ordering, but the new ordering is used to aid readability.)






share|cite|improve this answer























  • what if $aneq bneq cneq d$?
    – Enzo Creti
    Jan 5 at 19:12










  • they should be infinite
    – Enzo Creti
    Jan 5 at 20:37












  • no no ok thank you!
    – Enzo Creti
    Jan 5 at 20:40










  • @EnzoCreti : Even stronger: $|{a,b,c,d}| = 4$. $(4,2,3,5)$.
    – Eric Towers
    Jan 6 at 7:04
















2














This section of this Answer is responsive to the version of the Question which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d^2) = (2 a^3 + 2 b^3 + c^3 + d^3)$.



Notice that $c$ and $d$ are interchangeable, so we may impose $c leq d$.



One way to look at this is $b,c,d geq 1$ and $a =frac{2 b^3 + c^3 + d^3}{2 b^2 + c^2 + d^2}$, when that is an integer. This leads to the question, does that ever happen? Yes. Notice if $b = c = d$, this is $a = (4b^3)/(4 b^2) = b$, so $a = b = c = d$ is a solution for any choice of $a$.



begin{align*}
&(a,b,c,d) & text{giving} \
&(4,4,4,4) & 384 \
&(5,5,5,5) & 750 \
end{align*}



Cylindrical algebraic decomposition with variable order $c, d, a, b$, gives five cylindrical components. Exhibiting an integer point in a component or showing that a component has no integer points is not easy.





  • $c geq 1$, $d geq 1$, $a = b = frac{c^3 + d^3}{c^2 + d^2}$, when this is an integer. The fraction is always an integer when $c = d$ and is sometimes an integer otherwise. Examples: $(1,1,1,1)$, $(9,9,5,10)$


  • $d geq c geq 1$, $a > frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the least real root of $2x^3 -2ax^2 -a(c^2 +d^2) +c^3 + d^3$. Examples: $(9,10,5,5)$, $(17,18,2,14)$, and $(251,255,51,251)$.


  • $d geq c geq 1$, $a$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when those roots are integers. (In progress: determining whether those roots are ever integers.)


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(4,2,3,5)$ and $(5,2,2,6)$.


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the largest root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(5,4,1,6)$ and $(17,15,5,20)$.




The below is responsive to the first version of the Question, which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d) = (2 a^3 + 2 b^3 + c^3 + d)$.



Well, $a = b = c = d = 1$ is pretty obvious, giving $6$ on both sides.



One infinite family is $a=b=c = 1$, $d$ any positive integer, giving $d+5$. There don't seem to be more with $a = 1$, but I don't know how to prove that.



Here are eight more.
begin{align*}
&(a,b,c,d) & text{giving} \
&(2, 1, 7, 243) & 604 \
&(2, 1, 100, 979998) & 1980016 \
&(2, 2, 8, 384) & 928 \
&(2, 2, 68, 305184) & 619648 \
&(2, 3, 19, 6155) & 13084 \
&(2, 41, 1, 131117) & 268976 \
&(2, 41, 26, 147342) & 302776 \
&(2, 41, 27, 149343) & 306884
end{align*}



Cylindrical algebraic decomposition, using the variable ordering $b,c,a,d$ of your equation gives three solution families:





  • $a = b = c = 1$ and $d geq 1$, e.g., $(1,1,1,1)$ and $(1,1,1,108)$.


  • $b = 1$, $c geq 2$, $1 < a < frac{c^3+2}{c^2+2}$, $d = frac{c^3 - a c^2 - 2a + 2}{a-1}$, when $d$ is an integer, e.g., $(2,1,3,7)$ and $(109,1,126,2497)$.


  • $b geq 2$, $c geq 1$, $1 < a < frac{c^3+2b^3}{c^2+2b^2}$, $d = frac{c^4 - a c^2 + 2 b^3-2ab^2}{a-1}$, when $d$ is an integer, e.g., $(5,2,6,3)$ and $(3,3,100,485,000)$.


(The variable ordering $a,b,c,d$ gives more families, some of which appear to be empty, and most of which require bounds on $b$ and $c$ that are somewhat complicated algebraic functions of the prior variables. The previous version(s) of this answer used that ordering, but the new ordering is used to aid readability.)






share|cite|improve this answer























  • what if $aneq bneq cneq d$?
    – Enzo Creti
    Jan 5 at 19:12










  • they should be infinite
    – Enzo Creti
    Jan 5 at 20:37












  • no no ok thank you!
    – Enzo Creti
    Jan 5 at 20:40










  • @EnzoCreti : Even stronger: $|{a,b,c,d}| = 4$. $(4,2,3,5)$.
    – Eric Towers
    Jan 6 at 7:04














2












2








2






This section of this Answer is responsive to the version of the Question which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d^2) = (2 a^3 + 2 b^3 + c^3 + d^3)$.



Notice that $c$ and $d$ are interchangeable, so we may impose $c leq d$.



One way to look at this is $b,c,d geq 1$ and $a =frac{2 b^3 + c^3 + d^3}{2 b^2 + c^2 + d^2}$, when that is an integer. This leads to the question, does that ever happen? Yes. Notice if $b = c = d$, this is $a = (4b^3)/(4 b^2) = b$, so $a = b = c = d$ is a solution for any choice of $a$.



begin{align*}
&(a,b,c,d) & text{giving} \
&(4,4,4,4) & 384 \
&(5,5,5,5) & 750 \
end{align*}



Cylindrical algebraic decomposition with variable order $c, d, a, b$, gives five cylindrical components. Exhibiting an integer point in a component or showing that a component has no integer points is not easy.





  • $c geq 1$, $d geq 1$, $a = b = frac{c^3 + d^3}{c^2 + d^2}$, when this is an integer. The fraction is always an integer when $c = d$ and is sometimes an integer otherwise. Examples: $(1,1,1,1)$, $(9,9,5,10)$


  • $d geq c geq 1$, $a > frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the least real root of $2x^3 -2ax^2 -a(c^2 +d^2) +c^3 + d^3$. Examples: $(9,10,5,5)$, $(17,18,2,14)$, and $(251,255,51,251)$.


  • $d geq c geq 1$, $a$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when those roots are integers. (In progress: determining whether those roots are ever integers.)


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(4,2,3,5)$ and $(5,2,2,6)$.


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the largest root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(5,4,1,6)$ and $(17,15,5,20)$.




The below is responsive to the first version of the Question, which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d) = (2 a^3 + 2 b^3 + c^3 + d)$.



Well, $a = b = c = d = 1$ is pretty obvious, giving $6$ on both sides.



One infinite family is $a=b=c = 1$, $d$ any positive integer, giving $d+5$. There don't seem to be more with $a = 1$, but I don't know how to prove that.



Here are eight more.
begin{align*}
&(a,b,c,d) & text{giving} \
&(2, 1, 7, 243) & 604 \
&(2, 1, 100, 979998) & 1980016 \
&(2, 2, 8, 384) & 928 \
&(2, 2, 68, 305184) & 619648 \
&(2, 3, 19, 6155) & 13084 \
&(2, 41, 1, 131117) & 268976 \
&(2, 41, 26, 147342) & 302776 \
&(2, 41, 27, 149343) & 306884
end{align*}



Cylindrical algebraic decomposition, using the variable ordering $b,c,a,d$ of your equation gives three solution families:





  • $a = b = c = 1$ and $d geq 1$, e.g., $(1,1,1,1)$ and $(1,1,1,108)$.


  • $b = 1$, $c geq 2$, $1 < a < frac{c^3+2}{c^2+2}$, $d = frac{c^3 - a c^2 - 2a + 2}{a-1}$, when $d$ is an integer, e.g., $(2,1,3,7)$ and $(109,1,126,2497)$.


  • $b geq 2$, $c geq 1$, $1 < a < frac{c^3+2b^3}{c^2+2b^2}$, $d = frac{c^4 - a c^2 + 2 b^3-2ab^2}{a-1}$, when $d$ is an integer, e.g., $(5,2,6,3)$ and $(3,3,100,485,000)$.


(The variable ordering $a,b,c,d$ gives more families, some of which appear to be empty, and most of which require bounds on $b$ and $c$ that are somewhat complicated algebraic functions of the prior variables. The previous version(s) of this answer used that ordering, but the new ordering is used to aid readability.)






share|cite|improve this answer














This section of this Answer is responsive to the version of the Question which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d^2) = (2 a^3 + 2 b^3 + c^3 + d^3)$.



Notice that $c$ and $d$ are interchangeable, so we may impose $c leq d$.



One way to look at this is $b,c,d geq 1$ and $a =frac{2 b^3 + c^3 + d^3}{2 b^2 + c^2 + d^2}$, when that is an integer. This leads to the question, does that ever happen? Yes. Notice if $b = c = d$, this is $a = (4b^3)/(4 b^2) = b$, so $a = b = c = d$ is a solution for any choice of $a$.



begin{align*}
&(a,b,c,d) & text{giving} \
&(4,4,4,4) & 384 \
&(5,5,5,5) & 750 \
end{align*}



Cylindrical algebraic decomposition with variable order $c, d, a, b$, gives five cylindrical components. Exhibiting an integer point in a component or showing that a component has no integer points is not easy.





  • $c geq 1$, $d geq 1$, $a = b = frac{c^3 + d^3}{c^2 + d^2}$, when this is an integer. The fraction is always an integer when $c = d$ and is sometimes an integer otherwise. Examples: $(1,1,1,1)$, $(9,9,5,10)$


  • $d geq c geq 1$, $a > frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the least real root of $2x^3 -2ax^2 -a(c^2 +d^2) +c^3 + d^3$. Examples: $(9,10,5,5)$, $(17,18,2,14)$, and $(251,255,51,251)$.


  • $d geq c geq 1$, $a$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when those roots are integers. (In progress: determining whether those roots are ever integers.)


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the middle root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(4,2,3,5)$ and $(5,2,2,6)$.


  • $d geq c geq 1$, $r$ is the least real root of $8x^3 + 27(c^2+d^2)x - 27(c^3+d^3)$, $r < a < frac{c^3 + d^3}{c^2 + d^2}$, and $b$ is the largest root of $2 x^3 - 2 a x^2 -a(c^2 +d^2)+c^3+d^3$, when this root is an integer. Examples: $(5,4,1,6)$ and $(17,15,5,20)$.




The below is responsive to the first version of the Question, which asked for positive integral solutions to $a (2 a^2 + 2 b^2 + c^2 + d) = (2 a^3 + 2 b^3 + c^3 + d)$.



Well, $a = b = c = d = 1$ is pretty obvious, giving $6$ on both sides.



One infinite family is $a=b=c = 1$, $d$ any positive integer, giving $d+5$. There don't seem to be more with $a = 1$, but I don't know how to prove that.



Here are eight more.
begin{align*}
&(a,b,c,d) & text{giving} \
&(2, 1, 7, 243) & 604 \
&(2, 1, 100, 979998) & 1980016 \
&(2, 2, 8, 384) & 928 \
&(2, 2, 68, 305184) & 619648 \
&(2, 3, 19, 6155) & 13084 \
&(2, 41, 1, 131117) & 268976 \
&(2, 41, 26, 147342) & 302776 \
&(2, 41, 27, 149343) & 306884
end{align*}



Cylindrical algebraic decomposition, using the variable ordering $b,c,a,d$ of your equation gives three solution families:





  • $a = b = c = 1$ and $d geq 1$, e.g., $(1,1,1,1)$ and $(1,1,1,108)$.


  • $b = 1$, $c geq 2$, $1 < a < frac{c^3+2}{c^2+2}$, $d = frac{c^3 - a c^2 - 2a + 2}{a-1}$, when $d$ is an integer, e.g., $(2,1,3,7)$ and $(109,1,126,2497)$.


  • $b geq 2$, $c geq 1$, $1 < a < frac{c^3+2b^3}{c^2+2b^2}$, $d = frac{c^4 - a c^2 + 2 b^3-2ab^2}{a-1}$, when $d$ is an integer, e.g., $(5,2,6,3)$ and $(3,3,100,485,000)$.


(The variable ordering $a,b,c,d$ gives more families, some of which appear to be empty, and most of which require bounds on $b$ and $c$ that are somewhat complicated algebraic functions of the prior variables. The previous version(s) of this answer used that ordering, but the new ordering is used to aid readability.)







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited yesterday

























answered Jan 4 at 21:13









Eric TowersEric Towers

32.2k22267




32.2k22267












  • what if $aneq bneq cneq d$?
    – Enzo Creti
    Jan 5 at 19:12










  • they should be infinite
    – Enzo Creti
    Jan 5 at 20:37












  • no no ok thank you!
    – Enzo Creti
    Jan 5 at 20:40










  • @EnzoCreti : Even stronger: $|{a,b,c,d}| = 4$. $(4,2,3,5)$.
    – Eric Towers
    Jan 6 at 7:04


















  • what if $aneq bneq cneq d$?
    – Enzo Creti
    Jan 5 at 19:12










  • they should be infinite
    – Enzo Creti
    Jan 5 at 20:37












  • no no ok thank you!
    – Enzo Creti
    Jan 5 at 20:40










  • @EnzoCreti : Even stronger: $|{a,b,c,d}| = 4$. $(4,2,3,5)$.
    – Eric Towers
    Jan 6 at 7:04
















what if $aneq bneq cneq d$?
– Enzo Creti
Jan 5 at 19:12




what if $aneq bneq cneq d$?
– Enzo Creti
Jan 5 at 19:12












they should be infinite
– Enzo Creti
Jan 5 at 20:37






they should be infinite
– Enzo Creti
Jan 5 at 20:37














no no ok thank you!
– Enzo Creti
Jan 5 at 20:40




no no ok thank you!
– Enzo Creti
Jan 5 at 20:40












@EnzoCreti : Even stronger: $|{a,b,c,d}| = 4$. $(4,2,3,5)$.
– Eric Towers
Jan 6 at 7:04




@EnzoCreti : Even stronger: $|{a,b,c,d}| = 4$. $(4,2,3,5)$.
– Eric Towers
Jan 6 at 7:04











0














An obvious solution lies where a,b,c,d all equal 1 as:
$$1cdot(2cdot1^2+2cdot1^2+1^2+1) = (2cdot1^3+2cdot1^3+1^3+1)$$






share|cite|improve this answer








New contributor




Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.


















  • this seems a trivial solution
    – user631751
    Jan 4 at 20:58
















0














An obvious solution lies where a,b,c,d all equal 1 as:
$$1cdot(2cdot1^2+2cdot1^2+1^2+1) = (2cdot1^3+2cdot1^3+1^3+1)$$






share|cite|improve this answer








New contributor




Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.


















  • this seems a trivial solution
    – user631751
    Jan 4 at 20:58














0












0








0






An obvious solution lies where a,b,c,d all equal 1 as:
$$1cdot(2cdot1^2+2cdot1^2+1^2+1) = (2cdot1^3+2cdot1^3+1^3+1)$$






share|cite|improve this answer








New contributor




Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









An obvious solution lies where a,b,c,d all equal 1 as:
$$1cdot(2cdot1^2+2cdot1^2+1^2+1) = (2cdot1^3+2cdot1^3+1^3+1)$$







share|cite|improve this answer








New contributor




Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









share|cite|improve this answer



share|cite|improve this answer






New contributor




Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.









answered Jan 4 at 20:56









Peter ForemanPeter Foreman

2626




2626




New contributor




Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.





New contributor





Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.






Peter Foreman is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.












  • this seems a trivial solution
    – user631751
    Jan 4 at 20:58


















  • this seems a trivial solution
    – user631751
    Jan 4 at 20:58
















this seems a trivial solution
– user631751
Jan 4 at 20:58




this seems a trivial solution
– user631751
Jan 4 at 20:58










user631751 is a new contributor. Be nice, and check out our Code of Conduct.










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user631751 is a new contributor. Be nice, and check out our Code of Conduct.













user631751 is a new contributor. Be nice, and check out our Code of Conduct.












user631751 is a new contributor. Be nice, and check out our Code of Conduct.
















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