Number of meanders in which first bridge is 3.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =7a(5) =14a(6) =36a(7) =81a(8) =221a(9) =538a(10) =1530a(11) =3926a(12) =11510a(13) =30694a(14) =92114a(15) =252939a(16) =773259a(17) =2172830a(18) =6743122a(19) =19304190a(20) =60658606a(21) =176343390a(22) =559952824a(23) =1649008456a(24) =5283239596a(25) =15730575554a(26) =50789908772a(27) =152663683494