12063744domain: NAppears in sequencesa(n) = (n+1)*binomial(n+5, 5).at n=31A027810If X_1,...,X_n is a partition of a 2n-set X into 2-blocks then a(n) is equal to the number of 5-subsets of X containing none of X_i, (i=1,...n).at n=31A130811E.g.f.: Product_{k>=1} (1 + x^(4*k-3)/(4*k-3)) / (1 - x^(4*k-3)/(4*k-3)).at n=10A326863