OFFSET
2,3
COMMENTS
Conjecture: a(n) = 0 if p_n == 5 (mod 6).
For an odd prime p and integers c and d, Zhi-Wei Sun defined [c,d]_p to be the determinant of the p X p matrix with (i,j)-entry equal to the Legendre symbol ((i^2+c*i*j+d*j^2)/p). It is easy to see that [-c,d]_p = (-1/p)*[c,d]_p. Sun conjectured that for any integer c and nonzero integer d there are infinitely many odd primes p with [c,d]_p = 0. He also conjectured that if (c,d)_p (defined in the comments in A225611) is nonzero with d not divisible by p, then [c,d]_p/(c,d)_p equals (1-p)/(p-2) or (p-1)/2 according as p divides c^2-4*d or not. He also made some concrete conjectures, for example, [3,2]_p = 0 if p == 3 (mod 4); and [4,2]_p = 0 if p == 5 or 7 (mod 8).
On Aug 12 2013, Zhi-Wei Sun conjectured that for any odd prime p and integers c and d with d not divisible by p, if [c,d]_p is nonzero then its p-adic valuation (i.e., p-adic order) must be even.
LINKS
Zhi-Wei Sun, Table of n, a(n) for n = 2..50
Zhi-Wei Sun, On some determinants with Legendre symbol entries, preprint, arXiv:1308.2900 [math.NT], 2013-2019.
MATHEMATICA
a[n_]:=Det[Table[JacobiSymbol[i^2+3*i*j+3*j^2, Prime[n]], {i, 0, (Prime[n]-1)}, {j, 0, (Prime[n]-1)}]]
Table[a[n], {n, 2, 20}]
PROG
(PARI) a(n) = my(p=prime(n)); matdet(matrix(p, p, i, j, i--; j--; kronecker(i^2+3*i*j+3*j^2, p))); \\ Michel Marcus, Aug 25 2021
CROSSREFS
KEYWORD
sign
AUTHOR
Zhi-Wei Sun, Aug 12 2013
STATUS
approved