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 A321611 Number of permutations tau of {1,...,n} such that k^4 + tau(k)^4 is prime for every k = 1,...,n. 7
 1, 1, 1, 4, 4, 4, 4, 64, 16, 144, 144, 0, 144, 144, 289, 4356, 2916, 22500, 79524, 1887876, 313600, 3459600, 2985984, 50069776, 32353344, 2056803904, 237591396, 11713732900, 10265337124, 342040164964, 30744816964, 2507750953744, 378640854244, 53517915572836, 7415600385600, 230030730231696 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS Conjecture: Each term of the sequence is a positive square. See also A321597 and A321610 for similar conjectures. This conjecture fails for n = 12. The values of a(12),...,a(20) were first computed by the user MTson on Mathoverflow. Ilya Bogdanov has confirmed that a(n) is indeed a square. See answers and comments to Question 315351 on Mathoverflow. - Zhi-Wei Sun, Nov 17 2018 LINKS Table of n, a(n) for n=1..36. Zhi-Wei Sun, Primes arising from permutations (II), Question 315341 on Mathoverflow, Nov. 14, 2018. Zhi-Wei Sun, A mysterious connection between primes and squares, Question 315351 on Mathoverflow, Nov. 15, 2018. EXAMPLE a(3) = 1, and (1,3,2) is a permutation of {1,...,n} with 1^4 + 1^4 = 2, 2^4 + 3^4 = 97 and 3^4 + 2^4 = 97 all prime. MATHEMATICA V[n_]:=V[n]=Permutations[Table[i, {i, 1, n}]] Do[r=0; Do[Do[If[PrimeQ[i^4+Part[V[n], k][[i]]^4]==False, Goto[aa]], {i, 1, n}]; r=r+1; Label[aa], {k, 1, n!}]; Print[n, " ", r], {n, 1, 11}] PROG (PARI) a(n) = matpermanent(matrix(n, n, i, j, ispseudoprime(i^4 + j^4))); \\ Jinyuan Wang, Jun 13 2020 CROSSREFS Cf. A000040, A000583, A321597, A321610. Sequence in context: A264768 A201981 A309500 * A309756 A309767 A199500 Adjacent sequences: A321608 A321609 A321610 * A321612 A321613 A321614 KEYWORD nonn,more AUTHOR Zhi-Wei Sun, Nov 14 2018 EXTENSIONS a(12)-a(27) from Jud McCranie, Nov 17 2018 a(28)-a(29) from Jinyuan Wang, Jun 13 2020 a(30)-a(36) from Vaclav Kotesovec, Aug 19 2021 STATUS approved

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Last modified June 23 12:51 EDT 2024. Contains 373648 sequences. (Running on oeis4.)