OFFSET
0,3
COMMENTS
Two samples of size n are taken from an urn containing infinitely many marbles of n distinct colors. a(n)/n^(2*n) is the probability that the two samples match. That is, they contain the same number of each color of marbles without regard to order. - Geoffrey Critzer, Apr 19 2014
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..237
FORMULA
a(n) is coefficient of x^n in expansion of n!^2*(1 + x/1!^2 + x^2/2!^2 + x^3/3!^2 + ... + x^n/n!^2)^n. - Vladeta Jovovic, Jun 09 2000
a(n) ~ c * d^n * (n!)^2 / sqrt(n), where d = 2.1024237701057210364324371415246345951600138303179762223318873762632384990..., c = 0.487465475752598098146353111500372156824276600165331887960705498284416... - Vaclav Kotesovec, Jul 29 2014, updated Jul 10 2023
a(n) = n!^2 * [z^n] hypergeom([], [1], z)^n. - Peter Luschny, May 31 2017
MAPLE
b:= proc(n, i) option remember; `if`(n=0 or i=1, 1,
add(b(n-j, i-1)*binomial(n, j)^2, j=0..n))
end:
a:= n-> b(n$2):
seq(a(n), n=0..20); # Alois P. Heinz, Jul 21 2014
A033935:= proc(n) series(hypergeom([], [1], z)^n, z=0, n+1): n!^2*coeff(%, z, n) end: seq(A033935(n), n=0..16); # Peter Luschny, May 31 2017
MATHEMATICA
Table[nn=n; n!^2 Coefficient[Series[(Sum[x^k/k!^2, {k, 0, nn}])^n, {x, 0, nn}], x^n], {n, 1, 20}] (* Geoffrey Critzer, Apr 19 2014 *)
Flatten[{1, Table[n!^2*Coefficient[Series[BesselI[0, 2*Sqrt[x]]^n, {x, 0, n}], x^n], {n, 1, 20}]}] (* Vaclav Kotesovec, Jul 29 2014 *)
Table[SeriesCoefficient[HypergeometricPFQ[{}, {1}, x]^n, {x, 0, n}] n!^2, {n, 0, 16}] (* Peter Luschny, May 31 2017 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Warren D. Smith, Dec 11 1999
EXTENSIONS
More terms from James A. Sellers, Jun 01 2000 and Vladeta Jovovic, Jun 05 2000
a(0)=1 inserted by Alois P. Heinz, Jul 21 2014
STATUS
approved