a(n) = pg(n, 3) + pg(n, 4) + ... + pg(n, n) where pg(n, m) is the m-th n-th-order polygonal number.

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =25a(5) =69a(6) =154a(7) =300a(8) =531a(9) =875a(10) =1364a(11) =2034a(12) =2925a(13) =4081a(14) =5550a(15) =7384a(16) =9639a(17) =12375a(18) =15656a(19) =19550a(20) =24129a(21) =29469a(22) =35650a(23) =42756a(24) =50875a(25) =60099a(26) =70524a(27) =82250a(28) =95381a(29) =110025