a(n) = T(n, n-5), array T as in A055818.

Terms

a(0) =1a(1) =47a(2) =144a(3) =328a(4) =640a(5) =1131a(6) =1863a(7) =2910a(8) =4359a(9) =6311a(10) =8882a(11) =12204a(12) =16426a(13) =21715a(14) =28257a(15) =36258a(16) =45945a(17) =57567a(18) =71396a(19) =87728a(20) =106884a(21) =129211a(22) =155083a(23) =184902a(24) =219099a(25) =258135a(26) =302502a(27) =352724a(28) =409358a(29) =472995