Column 3 of triangle A055907.

Terms

a(0) =1a(1) =6a(2) =20a(3) =53a(4) =121a(5) =249a(6) =471a(7) =836a(8) =1409a(9) =2276a(10) =3544a(11) =5350a(12) =7862a(13) =11286a(14) =15866a(15) =21896a(16) =29721a(17) =39746a(18) =52436a(19) =68331a(20) =88047a(21) =112287a(22) =141841a(23) =177604a(24) =220577a(25) =271880a(26) =332752a(27) =404572a(28) =488860a(29) =587292