a(n) = n*(13*n + 1)/2.

Terms

a(0) =0a(1) =7a(2) =27a(3) =60a(4) =106a(5) =165a(6) =237a(7) =322a(8) =420a(9) =531a(10) =655a(11) =792a(12) =942a(13) =1105a(14) =1281a(15) =1470a(16) =1672a(17) =1887a(18) =2115a(19) =2356a(20) =2610a(21) =2877a(22) =3157a(23) =3450a(24) =3756a(25) =4075a(26) =4407a(27) =4752a(28) =5110a(29) =5481