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%I #3 Oct 02 2013 15:12:46
%S 4,15,57,765,1025,2097,4947,9189,103599,216927,4346128,1558269,1977777
%N a(n) = least composite m such that binomial(3m,m) mod m = 3^n.
%C Subsequence of A109641.
%t In[1]:= n = 1; Do[If[ !PrimeQ[k] && Mod[Binomial[3*k, k], k] == 3^n, Print[k]; n++ ], {k, 1, 10^4}]
%Y Cf. A080469, A109641.
%K nonn
%O 1,1
%A _Ryan Propper_, Aug 05 2005
%E Edited by Max Alekseyev, Nov 03 2009