3*h^2, where h is an odd integer not divisible by 3.

Terms

a(0) =3a(1) =75a(2) =147a(3) =363a(4) =507a(5) =867a(6) =1083a(7) =1587a(8) =1875a(9) =2523a(10) =2883a(11) =3675a(12) =4107a(13) =5043a(14) =5547a(15) =6627a(16) =7203a(17) =8427a(18) =9075a(19) =10443a(20) =11163a(21) =12675a(22) =13467a(23) =15123a(24) =15987a(25) =17787a(26) =18723a(27) =20667a(28) =21675a(29) =23763