a(n) is the number of (s(0),s(1),...,s(n)) such that every s(i) is a nonnegative integer, s(0) = 1, s(n) = 1, |s(1) - s(0)| = 1, |s(i) - s(i-1)| <= 1 for i >= 2. Also a(n) = T(n,n), where T is the array in A026120.

A026122

a(n) is the number of (s(0),s(1),...,s(n)) such that every s(i) is a nonnegative integer, s(0) = 1, s(n) = 1, |s(1) - s(0)| = 1, |s(i) - s(i-1)| <= 1 for i >= 2. Also a(n) = T(n,n), where T is the array in A026120.

Terms

    a(0) =2a(1) =4a(2) =11a(3) =28a(4) =74a(5) =196a(6) =525a(7) =1416a(8) =3846a(9) =10508a(10) =28864a(11) =79664a(12) =220818a(13) =614460a(14) =1715874a(15) =4807008a(16) =13506534a(17) =38052972a(18) =107477319a(19) =304261404a(20) =863188662a(21) =2453737132a(22) =6988033949a(23) =19935797080a(24) =56966012730a(25) =163026450132a(26) =467219178549

External references