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 A026861 #15 Apr 09 2023 23:04:58 %S A026861 1,5,22,95,411,1790,7855,34725,154573,692450,3120206,14135555, %T A026861 64356345,294341325,1351889910,6233399525,28845511125,133933280000, %U A026861 623811120960,2913924782375,13648296620445,64087737455725,301644762913977 %N A026861 T(2n,n+1), T given by A026747. %C A026861 a(n+1) = p(n+1) where p(x) is the unique degree-n polynomial such that p(k) = A002212(k+1) for k=0,1,...,n. - _Michael Somos_, Oct 07 2003 %C A026861 Number of skew Dyck paths of semilength n+1 containing at least one left step. - _David Scambler_, Jun 17 2013 %F A026861 a(n) = A002212(n+1) - A000108(n+1). - _David Scambler_, Jun 17 2013 %o A026861 (PARI) a(n)=if(n<1,0,subst(polinterpolate(Vec((1-3*x-sqrt(1-6*x+5*x^2+x^2*O(x^n)))/2)),x,n+1)) %Y A026861 Cf. A000108, A002212, A026747. %K A026861 nonn %O A026861 1,2 %A A026861 _Clark Kimberling_