OFFSET
1,7
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(7) = 2 because there are 2 such pairs, namely 14 = 3+13 = 7+7 with 3*13 == 5 (mod 14) and 7*7 == 7 (mod 14).
MAPLE
f:= proc(k) local P, i;
P:= select(t -> isprime(t) and isprime(2*k-t) and isprime(-t^2 mod (2*k)), [2, seq(i, i=3..k, 2)]);
nops(P);
end proc:
map(f, [$1..100]);
MATHEMATICA
a[n_] := Count[Select[Range[2, 2*n], PrimeQ], _?(# >= n && PrimeQ[2*n - #] && PrimeQ[Mod[#*(2*n - #), 2*n]] &)]; Array[a, 100] (* Amiram Eldar, Dec 28 2021 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
J. M. Bergot and Robert Israel, Dec 28 2021
STATUS
approved