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A331477 Number of n element multisets of n element multisets of an n-set.

%I A331477 #9 Aug 19 2021 11:48:38
%S A331477 1,1,6,220,73815,286243776,13949678575756,8800321588119330984,
%T A331477 73241437035618231162682185,8183604949527627465377060678018870,
%U A331477 12477686558866630120430437118910496237274716,263383624455446425760290611316122951917033704595478840
%N A331477 Number of n element multisets of n element multisets of an n-set.
%H A331477 Andrew Howroyd, <a href="/A331477/b331477.txt">Table of n, a(n) for n = 0..40</a>
%F A331477 a(n) = binomial(binomial(2*n - 1, n) + n - 1, n).
%e A331477 The a(2) = 6 multisets are:
%e A331477    {{1,1}, {1,1}},
%e A331477    {{1,1}, {1,2}},
%e A331477    {{1,1}, {2,2}},
%e A331477    {{1,2}, {1,2}},
%e A331477    {{1,2}, {2,2}},
%e A331477    {{2,2}, {2,2}}.
%t A331477 Table[Binomial[Binomial[2n-1,n]+n-1,n],{n,0,20}] (* _Harvey P. Dale_, Aug 19 2021 *)
%o A331477 (PARI) a(n)={binomial(binomial(2*n - 1, n) + n - 1, n)}
%Y A331477 Main diagonal of A331436.
%K A331477 nonn
%O A331477 0,3
%A A331477 _Andrew Howroyd_, Jan 17 2020