Number of symmetric meander shapes with 2n+1 crossings.

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =13a(5) =36a(6) =102a(7) =299a(8) =889a(9) =2698a(10) =8267a(11) =25684a(12) =80349a(13) =253872a(14) =806334a(15) =2580279a(16) =8290645a(17) =26794566a(18) =86881179a(19) =283034120a(20) =924521718a(21) =3031535538a(22) =9962795554