Centered triangular numbers: a(n) = 3*n*(n-1)/2 + 1.

Terms

a(0) =1a(1) =4a(2) =10a(3) =19a(4) =31a(5) =46a(6) =64a(7) =85a(8) =109a(9) =136a(10) =166a(11) =199a(12) =235a(13) =274a(14) =316a(15) =361a(16) =409a(17) =460a(18) =514a(19) =571a(20) =631a(21) =694a(22) =760a(23) =829a(24) =901a(25) =976a(26) =1054a(27) =1135a(28) =1219a(29) =1306