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

1,1

Comments

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

Links

Table of n, a(n) for n=1..44.

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 *)

Crossrefs

A001358 intersect A064826.

Cf. A000326, A001358, A064826.

Sequence in context: A046011 A038853 A038860 * A043391 A063358 A193398

Adjacent sequences: A120533 A120534 A120535 * A120537 A120538 A120539

Keyword

nonn

Author

Jonathan Vos Post, Aug 06 2006

Extensions

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

Status

approved