Doubly hexagonal numbers.

Terms

a(0) =0a(1) =1a(2) =66a(3) =435a(4) =1540a(5) =4005a(6) =8646a(7) =16471a(8) =28680a(9) =46665a(10) =72010a(11) =106491a(12) =152076a(13) =210925a(14) =285390a(15) =378015a(16) =491536a(17) =628881a(18) =793170a(19) =987715a(20) =1216020a(21) =1481781a(22) =1788886a(23) =2141415a(24) =2543640a(25) =3000025a(26) =3515226a(27) =4094091a(28) =4741660a(29) =5463165