a(n) = number of (s(0), s(1), ..., s(2n)) such that s(i) is a nonnegative integer and |s(i) - s(i-1)| = 1 for i = 1,2,...,n, s(0) = 2, s(2n) = 4. Also a(n) = T(2n,n-1), where T is the array defined in A026009.

A026013

a(n) = number of (s(0), s(1), ..., s(2n)) such that s(i) is a nonnegative integer and |s(i) - s(i-1)| = 1 for i = 1,2,...,n, s(0) = 2, s(2n) = 4. Also a(n) = T(2n,n-1), where T is the array defined in A026009.

Terms

    a(0) =1a(1) =4a(2) =15a(3) =55a(4) =200a(5) =726a(6) =2639a(7) =9620a(8) =35190a(9) =129200a(10) =476102a(11) =1760673a(12) =6533150a(13) =24319050a(14) =90795375a(15) =339929880a(16) =1275977670a(17) =4801199400a(18) =18106714050a(19) =68430306750a(20) =259129680264a(21) =983085703116

External references