Number of friezes of type B_n.

Terms

a(0) =2a(1) =6a(2) =21a(3) =75a(4) =273a(5) =1008a(6) =3762a(7) =14158a(8) =53635a(9) =204270a(10) =781378a(11) =2999906a(12) =11553234a(13) =44612760a(14) =172671925a(15) =669679793a(16) =2601913466a(17) =10125418060a(18) =39459828905a(19) =153977743500a(20) =601545298200