a(n) = 1*4*7 + 4*7*10 + 7*10*13 + ... (n terms).

Terms

a(0) =0a(1) =28a(2) =308a(3) =1218a(4) =3298a(5) =7250a(6) =13938a(7) =24388a(8) =39788a(9) =61488a(10) =91000a(11) =129998a(12) =180318a(13) =243958a(14) =323078a(15) =420000a(16) =537208a(17) =677348a(18) =843228a(19) =1037818a(20) =1264250a(21) =1525818a(22) =1825978a(23) =2168348a(24) =2556708a(25) =2995000a(26) =3487328a(27) =4037958a(28) =4651318a(29) =5331998