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 A121405 #6 Mar 30 2012 17:22:43
%S A121405 2,3,4,7,8,9,10,11,14,15,16,19,20,21,24,25,26,29,30,31,32,33,37,38,42,
%T A121405 43,44,45,46,49,50,54,55,59,60,61,65,66,67,72,77,80,81,84,89,94,95,96,
%U A121405 100,101,102,107,112,116,124,136,137,141,142,147,159
%N A121405 Numbers that are not the sum of distinct pentagonal numbers (A000326).
%C A121405 For sums of distinct generalized pentagonal numbers (A001318), only 4 and 11 are not representable.
%t A121405 nn=50; a=Table[n(3n-1)/2,{n,nn}]; t=Rest[CoefficientList[Series[Product[(1+x^a[[k]]), {k, nn}], {x,0,a[[ -1]]}], x]]; Flatten[Position[t,0]]
%Y A121405 Cf. A025524 (number of numbers not the sum of distinct n-th-order polygonal numbers)
%Y A121405 Cf. A007419 (largest number not the sum of distinct n-th-order polygonal numbers)
%Y A121405 Cf. A053614, A001422 (corresponding sequences for triangular and square numbers)
%K A121405 fini,nonn
%O A121405 1,1
%A A121405 _T. D. Noe_, Jul 28 2006