Number of symmetric reduced rearrangement maps.

Terms

a(0) =1a(1) =2a(2) =2a(3) =6a(4) =22a(5) =50a(6) =274a(7) =598a(8) =4486a(9) =9570a(10) =90914a(11) =191398a(12) =2201078a(13) =4593554a(14) =62012978a(15) =128619510a(16) =1993602406a(17) =4115824322a(18) =72026925634a(19) =148169675590