Crystal ball sequence for hexagonal close-packing.

Terms

a(0) =1a(1) =13a(2) =57a(3) =153a(4) =323a(5) =587a(6) =967a(7) =1483a(8) =2157a(9) =3009a(10) =4061a(11) =5333a(12) =6847a(13) =8623a(14) =10683a(15) =13047a(16) =15737a(17) =18773a(18) =22177a(19) =25969a(20) =30171a(21) =34803a(22) =39887a(23) =45443a(24) =51493a(25) =58057a(26) =65157a(27) =72813a(28) =81047a(29) =89879