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 A109642 #13 Jul 31 2025 08:43:46
%S A109642 4,15,57,765,1025,2097,4947,9189,103599,216927,4346128,1558269,
%T A109642 1977777,208510373,14493123,493262541,144228033
%N A109642 a(n) = least composite m such that binomial(3m,m) mod m = 3^n.
%C A109642 Subsequence of A109641.
%t A109642 In[1]:= n = 1; Do[If[ !PrimeQ[k] && Mod[Binomial[3*k, k], k] == 3^n, Print[k]; n++ ], {k, 1, 10^4}]
%Y A109642 Cf. A080469, A109641.
%K A109642 nonn,more
%O A109642 1,1
%A A109642 _Ryan Propper_, Aug 05 2005
%E A109642 Edited by _Max Alekseyev_, Nov 03 2009
%E A109642 a(14)-a(17) from _Chai Wah Wu_, Jul 30 2025