A129957(n) - n*(n-1)/2.

Terms

a(0) =1a(1) =1a(2) =8a(3) =24a(4) =59a(5) =115a(6) =202a(7) =322a(8) =485a(9) =693a(10) =956a(11) =1276a(12) =1663a(13) =2119a(14) =2654a(15) =3270a(16) =3977a(17) =4777a(18) =5680a(19) =6688a(20) =7811a(21) =9051a(22) =10418a(23) =11914a(24) =13549a(25) =15325a(26) =17252a(27) =19332a(28) =21575a(29) =23983