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 A136518 a(n) = A027907(2^n, n), where A027907 = triangle of trinomial coefficients. 1
 1, 2, 10, 112, 3620, 360096, 116950848, 129755798400, 507413158135840, 7132358041777380352, 364730093112968976177664, 68393665694364347188157159424, 47308574208170527265149009962117120 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(n) = [x^n] (1 + x + x^2)^(2^n), the coefficient of x^n in (1 + x + x^2)^(2^n). LINKS FORMULA O.g.f.: A(x) = Sum_{n>=0} log(1 + 2^n*x + 2^(2n)*x^2)^n / n!. EXAMPLE A(x) = 1 + 2x + 10x^2 + 112x^3 + 3620x^4 + 360096x^5 + ... A(x) = 1 + log(1+2x+4x^2) + log(1+4x+16x^2)^2/2! + log(1+8x+64x^2)^3/3! +... This is a special case of the more general statement: Sum_{n>=0} m^n * F(q^n*x)^b * log( F(q^n*x) )^n / n! = Sum_{n>=0} x^n * [y^n] F(y)^(m*q^n + b) where F(x) = 1+x+x^2, q=2, m=1, b=0. PROG (PARI) a(n)=polcoeff((1+x+x^2+x*O(x^n))^(2^n), n) (PARI) /* As coefficient x^n of Series: */ a(n)=polcoeff(sum(i=0, n, log(1+2^i*x+2^(2*i)*x^2 +x*O(x^n))^i/i!), n) CROSSREFS Cf. A027907, A136519. Sequence in context: A305854 A234296 A049505 * A168369 A317342 A226300 Adjacent sequences:  A136515 A136516 A136517 * A136519 A136520 A136521 KEYWORD nonn AUTHOR Paul D. Hanna, Jan 02 2008 STATUS approved

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Last modified August 11 00:32 EDT 2020. Contains 336403 sequences. (Running on oeis4.)