Number of symmetric ways of folding a strip of n labeled stamps.

Terms

a(0) =1a(1) =2a(2) =5a(3) =12a(4) =33a(5) =87a(6) =252a(7) =703a(8) =2105a(9) =6099a(10) =18689a(11) =55639a(12) =173423a(13) =526937a(14) =1664094a(15) =5137233a(16) =16393315a(17) =51255709a(18) =164951529a(19) =521138861a(20) =1688959630a(21) =5382512216a(22) =17547919924a(23) =56335234064a(24) =184596351277a(25) =596362337295