Numbers represented by hexagonal close-packing.

Terms

a(0) =0a(1) =3a(2) =6a(3) =8a(4) =9a(5) =11a(6) =12a(7) =15a(8) =17a(9) =18a(10) =19a(11) =20a(12) =21a(13) =22a(14) =25a(15) =27a(16) =29a(17) =30a(18) =31a(19) =32a(20) =33a(21) =34a(22) =35a(23) =36a(24) =37a(25) =39a(26) =41a(27) =43a(28) =44a(29) =45