Number of ways of 3-coloring an annulus consisting of n zones joined like a pearl necklace.

Terms

a(0) =0a(1) =6a(2) =6a(3) =18a(4) =30a(5) =66a(6) =126a(7) =258a(8) =510a(9) =1026a(10) =2046a(11) =4098a(12) =8190a(13) =16386a(14) =32766a(15) =65538a(16) =131070a(17) =262146a(18) =524286a(19) =1048578a(20) =2097150a(21) =4194306a(22) =8388606a(23) =16777218a(24) =33554430a(25) =67108866a(26) =134217726a(27) =268435458a(28) =536870910a(29) =1073741826