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 A188387 Central coefficient in (1 + (2^n+1)*x + 2^n*x^2)^n for n>=0. 1
 1, 3, 33, 1161, 140545, 63148833, 111254837505, 793938286762113, 23282575640347295745, 2812444483776375381074433, 1393909730376211388561041231873 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..36 FORMULA a(n) = Sum_{k=0..n} C(n,k)^2 * 2^(nk). PROG (PARI) {a(n)=polcoeff((1+(2^n+1)*x+2^n*x^2+x*O(x^n))^n, n)} (PARI) {a(n)=sum(k=0, n, binomial(n, k)^2*2^(n*k))} (MAGMA) /*1*/ P:=PolynomialRing(Integers()); [ Coefficients((1+(2^n+1)*x+2^n*x^2)^n)[n+1]: n in [0..10] ]; /*2*/ &cat[ [&+[ Binomial(n, k)^2*2^(n*k): k in [0..n]]]: n in [0..10] ];  //  Bruno Berselli, Mar 30 2011 CROSSREFS Cf. A187021. Sequence in context: A086894 A215948 A012487 * A113111 A118188 A194889 Adjacent sequences:  A188384 A188385 A188386 * A188388 A188389 A188390 KEYWORD nonn AUTHOR Paul D. Hanna, Mar 29 2011 STATUS approved

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