OFFSET
0,3
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..100
Moa Apagodu, David Applegate, N. J. A. Sloane, and Doron Zeilberger, Analysis of the Gift Exchange Problem, arXiv:1701.08394, 2017.
David Applegate and N. J. A. Sloane, The Gift Exchange Problem (arXiv:0907.0513, 2009)
FORMULA
a(n) = (5 + 3*n - 3*binomial(2*n+2, n+1) + A144660(n))/6. - Vaclav Kotesovec, Apr 04 2019
MAPLE
f:=n->add( add( add( (i+j+k)!/(3!*i!*j!*k!), i=1..n), j=1..n), k=1..n); [seq(f(n), n=0..20)];
MATHEMATICA
Table[Sum[Sum[Sum[(i+j+k)!/i!/j!/k!/6, {i, 1, n}], {j, 1, n}], {k, 1, n}], {n, 1, 30}]
Table[(5 + 3*n - 3*Binomial[2*n+2, n+1] + Sum[(1 + k + 2*n)! * HypergeometricPFQ[{1, -1 - k - n, -n}, {-1 - k - 2*n, -k - n}, 1] / ((1 + k + n)*k!*n!^2), {k, 0, n}]) / 6, {n, 0, 20}] (* Vaclav Kotesovec, Apr 04 2019 *)
PROG
(PARI) {a(n) = sum(i=1, n, sum(j=1, n, sum(k=1, n, (i+j+k)!/(6*i!*j!*k!))))} \\ Seiichi Manyama, Apr 03 2019
(PARI) {a(n) = sum(i=3, 3*n, i!*polcoef(sum(j=1, n, x^j/j!)^3, i))/6} \\ Seiichi Manyama, May 19 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
David Applegate and N. J. A. Sloane, Dec 15 2008, Jan 30 2009
STATUS
approved