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 A002243 #17 Nov 25 2014 10:54:38
%S A002243 1,2,3,5,6,8,12,15,20,23,33,78
%N A002243 Numbers that are not the sum of 3 distinct triangular numbers.
%t A002243 Complement[Range[100],Union[Total/@Subsets[Accumulate[Range[0,12]], {3}]]] (* _Harvey P. Dale_, Sep 02 2014 *)
%o A002243 (PARI) is(n)=for(c=ceil((sqrt(24*n-15)+3)/6),(sqrt(8*n-7)-1)/2,my(t=n-c*(c+1)/2);for(b=sqrtint(t-1)+1,min((sqrt(8*n+1)-1)/2,c-1), if(ispolygonal(t-b*(b+1)/2,3), return(0))));1
%o A002243 select(is,[1..100]) \\ _Charles R Greathouse IV_, Nov 25 2014
%Y A002243 Cf. A000217, A002244.
%K A002243 fini,full,nonn
%O A002243 1,2
%A A002243 _N. J. A. Sloane_, _Dan Hoey_