Duplicate of A020493.

Terms

a(0) =1a(1) =3a(2) =15a(3) =30a(4) =35a(5) =56a(6) =70a(7) =78a(8) =105a(9) =140a(10) =168a(11) =190a(12) =210a(13) =248a(14) =264a(15) =357a(16) =420a(17) =570a(18) =616a(19) =630