a(n) = ceiling( (n^2 + n + 2)/4 ).

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =8a(5) =11a(6) =15a(7) =19a(8) =23a(9) =28a(10) =34a(11) =40a(12) =46a(13) =53a(14) =61a(15) =69a(16) =77a(17) =86a(18) =96a(19) =106a(20) =116a(21) =127a(22) =139a(23) =151a(24) =163a(25) =176a(26) =190a(27) =204a(28) =218a(29) =233