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 A129256 Central coefficient of Product_{k=0..n} (1+k*x)^2. 5
 1, 2, 13, 144, 2273, 46710, 1184153, 35733376, 1251320145, 49893169050, 2232012515445, 110722046632560, 6032418472347265, 358103844593876654, 23007314730623658225, 1590611390957425536000, 117745011140615270168865 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS Vaclav Kotesovec, Table of n, a(n) for n = 0..354 FORMULA a(n) = (-1)^n*Sum_{k=0..n} Stirling1(n+1,k+1)*Stirling1(n+1,n-k+1). - Paul D. Hanna, Jul 16 2009 a(n) ~ c * d^n * (n-1)!, where d = A238261 = -(2*LambertW(-1,-exp(-1/2)/2))^2 / (1 + 2*LambertW(-1,-exp(-1/2)/2)) = 4.910814964568255..., c = 0.851946112888790982829578047527831525434714038256... . - Vaclav Kotesovec, Feb 10 2015 EXAMPLE This sequence equals the central terms of the triangle in which the g.f. of row n is (1+x)^2*(1+2x)^2*(1+3x)^2*...*(1+n*x)^2, as illustrated by: (1); 1,(2),1; 1,6,(13),12,4; 1,12,58,(144),193,132,36; 1,20,170,800,(2273),3980,4180,2400,576; 1,30,395,3000,14523,(46710),100805,143700,129076,65760,14400; ... MATHEMATICA Flatten[{1, Table[Coefficient[Expand[Product[(1+k*x), {k, 0, n}]^2], x^n], {n, 1, 20}]}] (* Vaclav Kotesovec, Feb 10 2015 *) PROG (PARI) a(n)=polcoeff(prod(k=0, n, 1+k*x)^2, n) Contribution from Paul D. Hanna, Jul 16 2009: (Start) (PARI) {Stirling1(n, k)=if(n==0, 1, n!*polcoeff(binomial(x, n), k))} {a(n)=(-1)^n*sum(k=0, n, Stirling1(n+1, k+1)*Stirling1(n+1, n-k+1))} (End) CROSSREFS Cf. A008275 (Stirling1 numbers), A238261. Sequence in context: A003326 A207493 A003581 * A046245 A178248 A107699 Adjacent sequences:  A129253 A129254 A129255 * A129257 A129258 A129259 KEYWORD nonn AUTHOR Paul D. Hanna, Apr 06 2007 STATUS approved

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Last modified February 28 08:28 EST 2020. Contains 332323 sequences. (Running on oeis4.)