Number of ways to choose two points on a centered hexagonal grid of size n.

Terms

a(0) =0a(1) =21a(2) =171a(3) =666a(4) =1830a(5) =4095a(6) =8001a(7) =14196a(8) =23436a(9) =36585a(10) =54615a(11) =78606a(12) =109746a(13) =149331a(14) =198765a(15) =259560a(16) =333336a(17) =421821a(18) =526851a(19) =650370a(20) =794430a(21) =961191a(22) =1152921a(23) =1371996a(24) =1620900a(25) =1902225a(26) =2218671a(27) =2573046a(28) =2968266a(29) =3407355