a(n) = p*(p-1)/2 for p = prime(n).

Terms

a(0) =1a(1) =3a(2) =10a(3) =21a(4) =55a(5) =78a(6) =136a(7) =171a(8) =253a(9) =406a(10) =465a(11) =666a(12) =820a(13) =903a(14) =1081a(15) =1378a(16) =1711a(17) =1830a(18) =2211a(19) =2485a(20) =2628a(21) =3081a(22) =3403a(23) =3916a(24) =4656a(25) =5050a(26) =5253a(27) =5671a(28) =5886a(29) =6328