138680365704domain: NAppears in sequencesa(n) = number of (s(0), s(1), ..., s(2*n-1)) such that s(i) is a nonnegative integer and |s(i) - s(i-1)| = 1 for i = 1,2,...,n, s(0) = 2, s(2*n-1) = 7. Also a(n) = T(2*n-1,n-3), where T is the array defined in A026009.at n=18A026018