Ordered (2,2)-selections from the multiset {1,1,2,2,3,3,...,n,n}.

Terms

a(0) =0a(1) =0a(2) =3a(3) =21a(4) =72a(5) =180a(6) =375a(7) =693a(8) =1176a(9) =1872a(10) =2835a(11) =4125a(12) =5808a(13) =7956a(14) =10647a(15) =13965a(16) =18000a(17) =22848a(18) =28611a(19) =35397a(20) =43320a(21) =52500a(22) =63063a(23) =75141a(24) =88872a(25) =104400a(26) =121875a(27) =141453a(28) =163296a(29) =187572