a(n) = floor((n^2 + 6n - 3)/4).

Terms

a(0) =1a(1) =3a(2) =6a(3) =9a(4) =13a(5) =17a(6) =22a(7) =27a(8) =33a(9) =39a(10) =46a(11) =53a(12) =61a(13) =69a(14) =78a(15) =87a(16) =97a(17) =107a(18) =118a(19) =129a(20) =141a(21) =153a(22) =166a(23) =179a(24) =193a(25) =207a(26) =222a(27) =237a(28) =253a(29) =269