OFFSET
1,1
COMMENTS
As is well known, for an odd prime p, b(p) is the smallest quadratic non-residue b modulo p if and only if b(p) is the smallest base b such that b^((p-1)/2) == -1 (mod p). Note that b(n) is always a prime.
Conjecture: If 2^((n-1)/2) == -1 (mod n), then b(n) = 2, where b(n) as above. This is true for odd primes n; is it for odd composites n? If so, then all composite numbers n such that 2^((n-1)/2) == -1 (mod n) are in this sequence.
It seems that, for defined pseudoprimes n (similar to the odd primes p),
b(n) is the smallest base b such that b^((n-1)/2) == -1 (mod n), although this is not required by their definition.
Note: a "non-residue" pseudoprime n is a strong pseudoprime to base b(n); the Jacobi symbol (b(n)/n) = -1, where b(n) is the smallest non-residue modulo n; such a pseudoprime n is not a Proth number, so n = k*2^m + 1 with odd k > 2^m.
Problem: are there infinitely many such numbers?
EXAMPLE
2^((3277-1)/2) == -1 (mod 3277), 3^((3281-1)/2) == -1 (mod 3281), ...
MATHEMATICA
residueQ[n_, m_] := Module[{ans = 0}, Do[If[Mod[k^2, m] == n, ans = True; Break[]], {k, 0, Floor[m/2]}]; ans]; A020649[n_] := Module[{m = 0}, While[ residueQ[m, n], m++]; m]; aQ[n_] := CompositeQ[n] && PowerMod[A020649[n], ((n - 1)/2), n] == n - 1; Select[Range[3, 110000, 2], aQ] (* Amiram Eldar, Apr 27 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Thomas Ordowski, Apr 27 2019
EXTENSIONS
More terms from Amiram Eldar, Apr 27 2019
STATUS
approved