OFFSET
0,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..150
FORMULA
a(n) = Sum_{k=0..n} A090595(k)*(n-k)!.
a(n) = Sum_{a+b+c+d = n} a!*b!*c!*d!.
G.f.: (Sum_{k>=0} k!*x^k)^4.
From G. C. Greubel, Dec 29 2019: (Start)
a(n) = (n+3)!*Sum_{k=0..n} Sum_{m=0..k} Sum_{j=0..m} Beta(k+3, n-k+1)*Beta(m+2, k-m+1)*Beta(j+1, m-j+1), where Beta(x,y) is the Beta function.
a(n) = Sum_{k=0..n} Sum_{m=0..k} Sum_{j=0..m} n!/(binomial(n,k) * binomial(k,m) * binomial(m,j)). (End)
MAPLE
seq( (n+3)!*add(add(add( Beta(k+3, n-k+1)*Beta(m+2, k-m+1)*Beta(j+1, m-j+1), j=0..m), m=0..k), k=0..n), n=0..20); # G. C. Greubel, Dec 29 2019
MATHEMATICA
Table[(n+3)!*Sum[Beta[k+3, n-k+1]*Beta[m+2, k-m+1]*Beta[j+1, m-j+1], {k, 0, n}, {m, 0, k}, {j, 0, m}], {n, 0, 20}] (* G. C. Greubel, Dec 29 2019 *)
PROG
(PARI) vector(21, n, my(b=binomial); sum(k=0, n-1, sum(m=0, k, sum(j=0, m, (n-1)!/(b(n-1, k)*b(k, m)*b(m, j)) )))) \\ G. C. Greubel, Dec 29 2019
(Magma) F:=Factorial; B:=Binomial; [ (&+[(&+[(&+[F(n)/(B(n, k)*B(k, m)*B(m, j)): j in [0..m]]): m in [0..k]]): k in [0..n]]): n in [0..20]]; // G. C. Greubel, Dec 29 2019
(Sage) b=binomial; [sum(sum(sum(factorial(n)/(b(n, k)*b(k, m)*b(m, j)) for j in (0..m)) for m in (0..k)) for k in (0..n)) for n in (0..20)] # G. C. Greubel, Dec 29 2019
(GAP) B:=Binomial;; List([0..20], n-> Sum([0..n], k-> Sum([0..k], m-> Sum([0..m], j-> Factorial(n)/(B(n, k)*B(k, m)*B(m, j)) )))); # G. C. Greubel, Dec 29 2019
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Philippe Deléham, Feb 05 2004
STATUS
approved