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 A135769 #7 Mar 31 2012 13:48:25
%S A135769 1,5,12,51,92,117,176,330,477,852,1080,4347,9801,13776,24512,27270,
%T A135769 39285,69876,88452,124272,137562,220992,268182,315792,354051,403782,
%U A135769 523626,612162,629532,699392,796797,919242,1119312,1158522,1190376
%N A135769 Pentagonal numbers > 0 which are not the difference of two other pentagonal numbers > 0.
%C A135769 A subsequence of A136113, obtained by omitting A136113(A135771(k)), k=1,2,3,... ; i.e. those which are not the difference of two larger pentagonal numbers, but the difference of a larger and a smaller pentagonal number.
%C A135769 The definition ("...two other...") excludes the case P(n) = P(m)-P(n), cf. comment by R. J. Mathar in A000326.
%H A135769 Donovan Johnson, <a href="/A135769/b135769.txt">Table of n, a(n) for n = 1..200</a>
%F A135769 P(n)=n*(3*n-1)/2 <=> n*(n-1/3) = (2/3)*P(n), thus m = P(n) <=> m = P([sqrt(2m/3)]+1)
%F A135769 and m = P(n) <=> 24m+1 = (6n-1)^2, useful for investigating the possibility of writing P(n)=P(n')+P(n"): this is possible whenever (6n-1)^2=(6n'-1)^2+(6n"-1)^2-1.
%e A135769 See A135768 for a list of P(n) which are in A136113 but not in A135769.
%o A135769 (PARI) P(n)=n*(3*n-1)/2
%o A135769 isPent(t)=P(sqrtint((t*2)\3)+1)==t
%o A135769 for( i=1,999,for( j=1,(P(i)-1)\3, isPent(P(i)+P(j))&j!=i&next(2)); print1(P(i)","))
%Y A135769 Cf. A000326, A136112-A136118, a(n) = A000326(A135768(n)), A135771 = A136112 \ A135768.
%K A135769 nonn
%O A135769 1,2
%A A135769 _R. J. Mathar_ and _M. F. Hasler_, Feb 07 2008