A120536 Semiprimes which are the sum of two pentagonal numbers (A000326) in exactly two different ways.

215, 381, 447, 766, 807, 1457, 1622, 1639, 1927, 2047, 2245, 2302, 2497, 3027, 3173, 3437, 3715, 3787, 4359, 4369, 4577, 4594, 4677, 4681, 5029, 5277, 5377, 5435, 5617, 5747, 5911, 6065, 6117, 6537, 6711, 6722, 6782, 7087, 7157, 7327, 7538, 7661, 7813, 7827

Offset: 1

Author

Jonathan Vos Post, Aug 06 2006

Keywords

• nonn

Comments

Semiprimes in A064826 Numbers which are the sum of two pentagonal numbers (A000326) in exactly two different ways.

Crossrefs

A001358 intersect A064826.

Cf. A000326, A001358, A064826.

Programs

Mathematica

T = Range[10^4]*0; q = # (3 # - 1)/2 &@ Range[81]; Do[ s = q[[i]] + q[[j]]; If[s <= 10^4, T[[s]]++], {i, Length[q]}, {j, i}]; Select[ Flatten@ Position[T, 2], PrimeOmega[#] == 2 &] (* Giovanni Resta, Jun 13 2016 *)

Extensions

a(14)-a(44) from Giovanni Resta, Jun 13 2016