a(n)=floor(3*n^2*(2+sqrt(3))).

Terms

a(0) =11a(1) =44a(2) =100a(3) =179a(4) =279a(5) =403a(6) =548a(7) =716a(8) =906a(9) =1119a(10) =1354a(11) =1612a(12) =1892a(13) =2194a(14) =2519a(15) =2866a(16) =3235a(17) =3627a(18) =4041a(19) =4478a(20) =4937a(21) =5418a(22) =5922a(23) =6448a(24) =6997a(25) =7568a(26) =8161a(27) =8777a(28) =9415a(29) =10076