Number of perfect matchings on a Möbius strip of width 4 and length n.

Terms

a(0) =11a(1) =37a(2) =71a(3) =252a(4) =539a(5) =1813a(6) =4271a(7) =13519a(8) =34276a(9) =103803a(10) =276119a(11) =813417a(12) =2226851a(13) =6455052a(14) =17965151a(15) =51604017a(16) =144948419a(17) =414258603a(18) =1169523076a(19) =3333192319a(20) =9436433171a(21) =26853404413a(22) =76139155439a(23) =216490730652a(24) =614339685971