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 A272988 Convolution of the sequence of derangement numbers A000166 with itself. 1
 1, 0, 2, 4, 19, 92, 552, 3832, 30453, 272552, 2713710, 29752156, 356133959, 4620985700, 64600445812, 967927029168, 15473320537001, 262864036323600, 4728905854617562, 89808092596277364, 1795480569403712699, 37693097921348983852, 829024574048725950016, 19063166411687276701736 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Vaclav Kotesovec, Table of n, a(n) for n = 0..440 FORMULA a(n) = Sum_{i=0..n} A000166(i)*A000166(n-i). G.f.: ( 1/(1 + x) + Sum_{k>=1} k^k*x^k/(1 + (k + 1)*x)^(k+1) )^2. - Ilya Gutkovskiy, Apr 13 2017 a(n) ~ 2*exp(-1)*n!. - Vaclav Kotesovec, Apr 13 2017 EXAMPLE For n = 4, we get 1*9 + 0*2 + 1*1 + 2*0 + 9*1 = 19. MATHEMATICA Table[Sum[Subfactorial[k] Subfactorial[n - k], {k, 0, n}], {n, 0, 30}] (* Emanuele Munarini, Oct 06 2017 *) CROSSREFS Cf. A000166. Sequence in context: A064228 A226891 A289269 * A168246 A212923 A058130 Adjacent sequences:  A272985 A272986 A272987 * A272989 A272990 A272991 KEYWORD easy,nonn AUTHOR J. C. George, May 12 2016 STATUS approved

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Last modified August 18 14:30 EDT 2018. Contains 313832 sequences. (Running on oeis4.)