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 A196990 #7 Mar 30 2012 17:24:46 %S A196990 1,3,5,7,8,9,11,13,14,15,16,17,19,20,21,22,23,24,25,26,27,29,31,32,33, %T A196990 34,35,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,55,56,57,58, %U A196990 59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76 %N A196990 Numbers that are not the sum of two powers of 3. %C A196990 Complement of A055235 (sums of two powers of 3) %e A196990 a(2)=3 because 3 cannot be expressed as the sum of two powers of 3 %Y A196990 Cf. A055235, A072823. %K A196990 nonn %O A196990 1,2 %A A196990 _Jeremy Gardiner_, Oct 08 2011