Triangular numbers with square pyramidal indices.

Terms

a(0) =0a(1) =1a(2) =15a(3) =105a(4) =465a(5) =1540a(6) =4186a(7) =9870a(8) =20910a(9) =40755a(10) =74305a(11) =128271a(12) =211575a(13) =335790a(14) =515620a(15) =769420a(16) =1119756a(17) =1594005a(18) =2224995a(19) =3051685a(20) =4119885a(21) =5483016a(22) =7202910a(23) =9350650a(24) =12007450a(25) =15265575a(26) =19229301a(27) =24015915a(28) =29756755a(29) =36598290