This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A117961 #9 Nov 21 2013 12:48:57
%S A117961 6,15,45,91,231,325,561,703,1035,1653,1891,2701,3321,3655,4371,5565,
%T A117961 6903,7381,8911,10011,10585,12403,13695,15753,18721,20301,21115,22791,
%U A117961 23653,25425,32131,34191,37401,38503,44253,45451,49141,52975,55611
%N A117961 Hexagonal numbers with prime indices.
%C A117961 See also: A034953 Triangular numbers (A000217) with prime indices. A001248 Squares of primes. A116995 Pentagonal numbers with prime indices. A000384 Hexagonal numbers: n(2n-1). There are no prime hexagonal numbers. The n-th Hexagonal number A000384(n) = n*(2*n-1) is semiprime iff both n and 2*n-1 are prime iff A000384(n) is an element of A001358 iff n is an element of A005382.
%F A117961 a(n) = A000040(n)*(2*A000040(n)-1). a(n) = A000384(prime(n)). a(n) = number of divisors of 12^(prime(n)-1) = A000005(A001021(A000040(n)-1)).
%t A117961 With[{hex=Table[n(2n-1),{n,250}]},Flatten[Table[Take[hex,{Prime[n]}],{n, 40}]]] (* _Harvey P. Dale_, Dec 04 2011 *)
%Y A117961 Cf. A000005, A000040, A000384, A001021, A034953, A001248, A116995.
%K A117961 easy,nonn,less
%O A117961 1,1
%A A117961 _Jonathan Vos Post_, Apr 05 2006