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 A081291 #6 Apr 28 2020 09:20:53 %S A081291 0,3,6,7,8,14,15,16,17,18,19,20,21,22,37,38,39,40,41,42,43,44,45,46, %T A081291 47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,107,108,109, %U A081291 110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128 %N A081291 Complement of A072795. %C A081291 This gives positions of those terms in A063171 which begin with digits 11... Also the elements of table A072764 which do not occur in the leftmost column. See the comment at A081292. %F A081291 a(0)=0, a(n) = n + A014137(A081288(n)-1). %F A081291 a(n) = A080300(A081292(n)) = A081289(n) + n - A081290(n). %o A081291 (PARI) A014137(n) = sum(k=0, n, binomial(2*k, k)/(k+1);); %o A081291 A081288(n) = my(i=0); while(binomial(2*i, i)/(i+1) <= n, i++); i; %o A081291 a(n) = if (n==0, 0, n + A014137(A081288(n)-1)); \\ _Michel Marcus_, Apr 28 2020 %Y A081291 Cf. A072795, A014137, A081288. %Y A081291 Cf. A080300, A081289, A081290, A081292. %K A081291 nonn %O A081291 0,2 %A A081291 _Antti Karttunen_, Mar 17 2003