Triply triangular numbers.

Terms

a(0) =0a(1) =1a(2) =21a(3) =231a(4) =1540a(5) =7260a(6) =26796a(7) =82621a(8) =222111a(9) =536130a(10) =1186570a(11) =2445366a(12) =4747821a(13) =8763391a(14) =15487395a(15) =26357430a(16) =43398586a(17) =69401871a(18) =108140571a(19) =164629585a(20) =245433090a(21) =359026206a(22) =516216646a(23) =730632651a(24) =1019283825a(25) =1403201800a(26) =1908167976a(27) =2565535896a(28) =3413156131