a(n) is the solution to the postage stamp problem with 4 denominations and n stamps.

Terms

a(0) =4a(1) =12a(2) =24a(3) =44a(4) =71a(5) =114a(6) =165a(7) =234a(8) =326a(9) =427a(10) =547a(11) =708a(12) =873a(13) =1094a(14) =1383a(15) =1650a(16) =1935a(17) =2304a(18) =2782a(19) =3324a(20) =3812a(21) =4368a(22) =5130a(23) =5892a(24) =6745a(25) =7880a(26) =8913a(27) =9919a(28) =11081a(29) =12376