Spiral of triangles around a hexagon.

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =5a(5) =7a(6) =9a(7) =12a(8) =16a(9) =21a(10) =28a(11) =37a(12) =49a(13) =65a(14) =86a(15) =114a(16) =151a(17) =200a(18) =265a(19) =351a(20) =465a(21) =616a(22) =816a(23) =1081a(24) =1432a(25) =1897a(26) =2513a(27) =3329a(28) =4410a(29) =5842