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 A201823 #9 Mar 30 2012 18:37:33 %S A201823 1,1,55,300830572,1414282077098335379544565517191 %N A201823 Terms of A001764 not divisible by 3, where A001764 enumerates ternary trees. %C A201823 The number of digits for the terms begin: %C A201823 [1, 1, 2, 9, 31, 97, 298, 902, 2715, 8155, 24478, 73446, 220354, ...]. %F A201823 a(n) = A001764( (3^n-1)/2 ) = binomial( 3*(3^n-1)/2, (3^n-1)/2 ) / 3^n. %F A201823 a(n) == 1 (mod 3). %e A201823 Sequence A001764 begins: [1, 1, 3, 12, 55, 273, 1428, 7752, 43263, 246675, ...], %e A201823 where A001764(n) == 1 (mod 3) at positions: [0, 1, 4, 13, 40, 121, 364, 1093, ...]. %o A201823 (PARI) a(n)=binomial(3*(3^n-1)/2,(3^n-1)/2)/3^n %Y A201823 Cf. A038003, A001764. %K A201823 nonn %O A201823 0,3 %A A201823 _Paul D. Hanna_, Dec 05 2011