A026570 a(n) = A026568(n,n-1), also a(n) = number of integer strings s(0),...,s(n) counted by A026568 such that s(n)=1.

1, 1, 4, 7, 20, 43, 111, 259, 648, 1565, 3885, 9533, 23662, 58547, 145630, 362151, 903110, 2253615, 5633359, 14094035, 35304658, 88511733, 222115782, 557819793, 1401987930, 3526066273, 8874034647, 22346581133, 56304982154

Offset: 1

Author

Clark Kimberling

Keywords

• nonn

Comments

Also a(n) = T'(n,n-1), T' given by A026584. Also a(n) = number of integer strings s(0),...,s(n) counted by T' such that s(n)=1.

Crossrefs

Cf. A026568, A026569, A026584, A026585.

Formula

Conjecture: (n+1)*a(n) -2*n*a(n-1) +(-3*n-1)*a(n-2) +2*(2*n-3)*a(n-3)=0. - R. J. Mathar, Jun 23 2013

If recurrence is correct then a(n) = (A026569(n+1)-A026569(n))/2 = A026585(n+1)/2. - Mark van Hoeij, Nov 29 2024