OFFSET
0,3
LINKS
Andrew Howroyd, Table of n, a(n) for n = 0..40
FORMULA
a(n) = binomial(binomial(2*n - 1, n) + n - 1, n).
EXAMPLE
The a(2) = 6 multisets are:
{{1,1}, {1,1}},
{{1,1}, {1,2}},
{{1,1}, {2,2}},
{{1,2}, {1,2}},
{{1,2}, {2,2}},
{{2,2}, {2,2}}.
MATHEMATICA
Table[Binomial[Binomial[2n-1, n]+n-1, n], {n, 0, 20}] (* Harvey P. Dale, Aug 19 2021 *)
PROG
(PARI) a(n)={binomial(binomial(2*n - 1, n) + n - 1, n)}
CROSSREFS
KEYWORD
nonn
AUTHOR
Andrew Howroyd, Jan 17 2020
STATUS
approved