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 A227609 Determinant of the (p_n-1)/2 X (p_n-1)/2 matrix with (i,j)-entry being the Legendre symbol((i^2+j^2)/p_n), where p_n is the n-th prime. 9
 -1, 1, -4, -16, -27, 441, -1024, -1024, 34445, -13778944, 82719025, 48841786125, -67649929216, -564926611456, -153908556861703, -25481517249593344, 2456184022341328125, -399780402627654713344, -14448269983744, -214168150727821285287075 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,3 COMMENTS Conjecture: p_n never divides a(n), and moreover -a(n) is a quadratic residue mod p_n. Zhi-Wei Sun also made the following conjecture:   Let p be any odd prime. For each integer d let S(d,p) be the determinant of the (p-1)/2 X (p-1)/2 matrix whose (i,j)-entry is the Legendre symbol ((i^2+d*j^2)/p). If d is a quadratic residue mod p, then so is -S(d,p). If d is a quadratic non-residue mod p, then we have S(d,p) = 0. These were proved in version 9 of arXiv:1308.2900 (2018). In addition, the author has the following new conjecture. Conjecture: For any prime p == 3 (mod 4), the number -S(1,p) is a positive square divisible by 2^((p-3)/2), i.e., -S(1,p) = (2^((p-3)/4)*m)^2 for some positive integer m. - Zhi-Wei Sun, Sep 09 2018 LINKS Alois P. Heinz, Table of n, a(n) for n = 2..100 Zhi-Wei Sun, On some determinants with Legendre symbol entries, preprint, arXiv:1308.2900 [math.NT], 2013-2019. Zhi-Wei Sun, Is -det[Legendre(i^2+j^2,p)]_{i,j=1,...,(p-1)/2} always a square for each prime p == 3 (mod 4)?, Question 310192 in MathOverflow, Sept. 9, 2018. EXAMPLE a(2) = -1 since the Legendre symbol ((1^2 + 1^2)/3) is -1. MAPLE with(numtheory): with(LinearAlgebra): a:= n-> Determinant(Matrix((ithprime(n)-1)/2, (i, j)->         jacobi(i^2+j^2, ithprime(n)))): seq(a(n), n=2..20);  # Alois P. Heinz, Jul 18 2013 MATHEMATICA a[n_]:=Det[Table[JacobiSymbol[i^2+j^2, Prime[n]], {i, 1, (Prime[n]-1)/2}, {j, 1, (Prime[n]-1)/2}]] Table[a[n], {n, 2, 20}] PROG (PARI) a(n) = my(p=prime(n)); matdet(matrix((p-1)/2, (p-1)/2, i, j, kronecker(i^2+j^2, p))); \\ Michel Marcus, Aug 25 2021 CROSSREFS Cf. A179071, A179072, A227968, A227971. Sequence in context: A271936 A046358 A046366 * A219338 A275211 A046361 Adjacent sequences:  A227606 A227607 A227608 * A227610 A227611 A227612 KEYWORD sign AUTHOR Zhi-Wei Sun, Jul 17 2013 STATUS approved

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Last modified November 29 21:32 EST 2021. Contains 349416 sequences. (Running on oeis4.)