a(n) = 2*(n+1) + 3*n + ... + (k+1)*(n+2-k), where k = floor((n+1)/2).

Terms

a(0) =4a(1) =6a(2) =17a(3) =22a(4) =43a(5) =52a(6) =86a(7) =100a(8) =150a(9) =170a(10) =239a(11) =266a(12) =357a(13) =392a(14) =508a(15) =552a(16) =696a(17) =750a(18) =925a(19) =990a(20) =1199a(21) =1276a(22) =1522a(23) =1612a(24) =1898a(25) =2002a(26) =2331a(27) =2450a(28) =2825a(29) =2960