A026027 a(n) = number of (s(0), s(1), ..., s(2n-1)) such that s(i) is a nonnegative integer and |s(i) - s(i-1)| = 1 for i = 1,2,...,n, s(0) = 3, s(2n-1) = 6. Also a(n) = T(2n-1,n-2), where T is defined in A026022.
1, 5, 21, 84, 329, 1274, 4900, 18768, 71706, 273581, 1043119, 3976700, 15163785, 57848910, 220830360, 843621600, 3225477150, 12342901410, 47274639594, 181230395304, 695384769002, 2670587146260, 10265249568536, 39491671919840
Offset: 2
Keywords
Formula
C(2n-1, n-2) - C(2n-1, n-6). G.f.: (1+x^2C^4)*C^5, where C=(1-sqrt(1-4x))/(2x). - Ralf Stephan, Jan 09 2005
Conjecture: (13*n+118)*(n+5)*a(n) +(13*n^2-969*n-3490)*a(n-1) +2*(-203*n^2+1080*n+758)*a(n-2) +4*(73*n-182)*(2*n-5)*a(n-3)=0. - R. J. Mathar, Jun 22 2013