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 A206156 a(n) = Sum_{k=0..n} binomial(n,k)^(2*k). 4
 1, 2, 6, 92, 5410, 1400652, 2687407464, 18947436116184, 536104663173431874, 130559883231879141946580, 136031455187223511721647272376, 483565526783420050082035900177878504, 14487924180895151383693101563813954330590756 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Ignoring initial term a(0), equals the logarithmic derivative of A206155. LINKS FORMULA Limit n->infinity a(n)^(1/n^2) = r^(2*r^2/(1-2*r)) = 2.3520150420944489879258119..., where r = 0.70350607643066243... (see A220359) is the root of the equation (1-r)^(2*r-1) = r^(2*r). - Vaclav Kotesovec, Mar 03 2014 EXAMPLE L.g.f.: L(x) = 2*x + 6*x^2/2 + 92*x^3/3 + 5410*x^4/4 + 1400652*x^5/5 +... where exponentiation yields A206155: exp(L(x)) = 1 + 2*x + 5*x^2 + 38*x^3 + 1425*x^4 + 283002*x^5 + 448468978*x^6 +... Illustration of initial terms: a(1) = 1^0 + 1^2 = 2; a(2) = 1^0 + 2^2 + 1^4 = 6; a(3) = 1^0 + 3^2 + 3^4 + 1^6 = 92; a(4) = 1^0 + 4^2 + 6^4 + 4^6 + 1^8 = 5410; a(5) = 1^0 + 5^2 + 10^4 + 10^6 + 5^8 + 1^10 = 1400652; ... MATHEMATICA Table[Sum[Binomial[n, k]^(2*k), {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Mar 03 2014 *) PROG (PARI) {a(n)=sum(k=0, n, binomial(n, k)^(2*k))} for(n=0, 16, print1(a(n), ", ")) CROSSREFS Cf. A206155 (exp), A184731, A206154, A206158, A206152, A220359. Sequence in context: A177861 A218151 A007188 * A229052 A280117 A129364 Adjacent sequences:  A206153 A206154 A206155 * A206157 A206158 A206159 KEYWORD nonn AUTHOR Paul D. Hanna, Feb 04 2012 STATUS approved

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Last modified December 3 16:20 EST 2020. Contains 338906 sequences. (Running on oeis4.)