G.f.: A(x) = exp( Sum_{n>=1} (2^n + 1)^n * x^n/n ), a power series in x with integer coefficients.
1, 3, 17, 285, 21747, 7894143, 12593691755, 84961748935779
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Wed Feb 11 18:24:22 CET 2009 [seqfan] Re: Peculiar Integer Recurrences ... Proof?