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 A211898 G.f.: exp( Sum_{n>=1} (2^n - (-1)^n)^n * x^n/n ). 1
 1, 3, 9, 261, 13419, 7867287, 10444212819, 84955235950827, 2235017786095822257, 273416315791427558035965, 125533366255776787874473759857, 242979442003484538229530424638338553, 1852958949086213206247388599213928431454549 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS CONJECTURE: the highest power of 3 dividing a(n) equals 3^A089792(n) for n>=0; that is, n!*a(n)/3^n is an integer not divisible by 3 for n>=0. Given g.f. A(x), note that A(x)^(1/3) is not an integer series. LINKS FORMULA a(n) == 3 (mod 6) for n>0. EXAMPLE G.f.: A(x) = 1 + 3*x + 9*x^2 + 261*x^3 + 13419*x^4 + 7867287*x^5 +... such that log(A(x)) = 3*x + 3^2*x^2/2 + 9^3*x^3/3 + 15^4*x^4/4 + 33^5*x^5/5 + 63^6*x^6/6 + 129^7*x^7/7 + 255^8*x^8/8 +...+ (2^n - (-1)^n)^n*x^n/n +... PROG (PARI) {a(n)=polcoeff(exp(sum(k=1, n, (2^k-(-1)^k)^k*x^k/k)+x*O(x^n)), n)} for(n=0, 20, print1(a(n), ", ")) CROSSREFS Cf. A211897, A155200, A089792. Sequence in context: A027891 A073889 A332586 * A318970 A132516 A328125 Adjacent sequences:  A211895 A211896 A211897 * A211899 A211900 A211901 KEYWORD nonn AUTHOR Paul D. Hanna, Apr 25 2012 STATUS approved

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Last modified June 12 14:32 EDT 2021. Contains 344957 sequences. (Running on oeis4.)