a(n) = binomial(n+1, 2) + n - 1 = n*(n+3)/2 - 1.

Terms

a(0) =1a(1) =4a(2) =8a(3) =13a(4) =19a(5) =26a(6) =34a(7) =43a(8) =53a(9) =64a(10) =76a(11) =89a(12) =103a(13) =118a(14) =134a(15) =151a(16) =169a(17) =188a(18) =208a(19) =229a(20) =251a(21) =274a(22) =298a(23) =323a(24) =349a(25) =376a(26) =404a(27) =433a(28) =463a(29) =494