a(n)=T(n,n), array T as in A049723.

Terms

a(0) =1a(1) =6a(2) =15a(3) =35a(4) =56a(5) =88a(6) =121a(7) =162a(8) =214a(9) =267a(10) =331a(11) =396a(12) =461a(13) =553a(14) =634a(15) =734a(16) =827a(17) =931a(18) =1040a(19) =1153a(20) =1289a(21) =1414a(22) =1554a(23) =1691a(24) =1832a(25) =2004a(26) =2161a(27) =2329a(28) =2502a(29) =2678