Number of integer sequences [y(1),...,y(n)] such that |y(i)-y(i+1)|<=1, 1=y(1)<=y(i)<=y(n).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =22a(6) =56a(7) =146a(8) =387a(9) =1039a(10) =2819a(11) =7717a(12) =21287a(13) =59105a(14) =165038a(15) =463089a(16) =1304945a(17) =3690970a(18) =10474213a(19) =29811119a(20) =85070660a(21) =243340754a(22) =697571153a(23) =2003636629a(24) =5765473829a(25) =16617825248a(26) =47971281916a(27) =138677926537