Number of solutions to a 4 X n Ring-Ring puzzle on an empty grid.

Terms

a(0) =1a(1) =0a(2) =2a(3) =1a(4) =8a(5) =12a(6) =45a(7) =98a(8) =292a(9) =725a(10) =2004a(11) =5210a(12) =14051a(13) =37126a(14) =99336a(15) =263991a(16) =704622a(17) =1876566a(18) =5005327a(19) =13341292a(20) =35579622a(21) =94866875a(22) =252998042a(23) =674677856a(24) =1799331865a(25) =4798687520a(26) =12798199466a(27) =34133165825a(28) =91035612400a(29) =242799614636