n(n+Z(n)), where Z( ) is the Narayana-Zidek-Capell sequence (A002083).

Terms

a(0) =0a(1) =2a(2) =6a(3) =12a(4) =24a(5) =40a(6) =72a(7) =126a(8) =240a(9) =459a(10) =940a(11) =1936a(12) =4104a(13) =8671a(14) =18508a(15) =39300a(16) =83616a(17) =177055a(18) =374652a(19) =789811a(20) =1662400a(21) =3488877a(22) =7309588a(23) =15279406a(24) =31886928a(25) =66422275a(26) =138157708a(27) =286924599a(28) =595102144a(29) =1232672869