Number of order-6 ribbon tilings for a 6 X n strip.

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =120a(5) =720a(6) =2520a(7) =9240a(8) =35490a(9) =142758a(10) =601167a(11) =2648967a(12) =10532752a(13) =42054602a(14) =168941427a(15) =684260967a(16) =2800315185a(17) =11604074418a(18) =47073995851a(19) =190962943230a(20) =775113276195