Crystal ball sequence for diamond.

Terms

a(0) =1a(1) =5a(2) =17a(3) =41a(4) =83a(5) =147a(6) =239a(7) =363a(8) =525a(9) =729a(10) =981a(11) =1285a(12) =1647a(13) =2071a(14) =2563a(15) =3127a(16) =3769a(17) =4493a(18) =5305a(19) =6209a(20) =7211a(21) =8315a(22) =9527a(23) =10851a(24) =12293a(25) =13857a(26) =15549a(27) =17373a(28) =19335a(29) =21439