OFFSET
1,18
EXAMPLE
18 can be written in two ways as the sum of two incongruent primes modulo 3: 18 = 5 + 13 (5 = 2 mod 3; 13 = 1 mod 3) and 18 = 7 + 11 (order of addition is ignored). Hence a(18) = 2.
MATHEMATICA
f[n_] := Module[{a, d, i}, a = {}; u = Floor[n/2]; For[i = 1, i <= u, i++, If[PrimeQ[i] && PrimeQ[n - i] && Mod[i, 3] != Mod[n - i, 3], a = Append[a, {n, i, n - i}]]]; a]; Table[Length[f[n]], {n, 1, 200}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Joseph L. Pe, Dec 13 2002
STATUS
approved