a(n) is the number of length-n ballot sequences whose reversals are also ballot sequences.

A338418

a(n) is the number of length-n ballot sequences whose reversals are also ballot sequences.

Terms

    a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =5a(6) =11a(7) =23a(8) =50a(9) =118a(10) =283a(11) =731a(12) =1921a(13) =5370a(14) =14884a(15) =43118a(16) =130509a(17) =397490a(18) =1266010a(19) =4077494a(20) =13435445a(21) =45687806a(22) =157483627a(23) =551205779a(24) =1979511990

External references