A027558 Number of 3-unbalanced strings of length n (= 2^n - A027557(n)).

0, 0, 0, 0, 2, 6, 20, 50, 126, 288, 654, 1422, 3068, 6474, 13574, 28112, 57942, 118566, 241788, 490914, 994334, 2008512, 4050398, 8154014, 16396668, 32935194, 66104982, 132586416, 265792070, 532579830, 1066790972

Offset: 0

Author

R. K. Guy, David Callan

Keywords

• nonn

Comments

a(n) is also the number of different ways to fold down a folding yardstick with n sticks (stick length is one) to form an equilateral tetragon (parallelogram) with side length one. - Frank Hollstein, May 01 2019

Links

• Index entries for linear recurrences with constant coefficients, signature (3,1,-8,2,4).

Formula

a(n) = 2*A005674(n).

G.f.: -2*x^4/((2*x-1)*(2*x^2-1)*(x^2+x-1)). [Maksym Voznyy (voznyy(AT)mail.ru), Aug 12 2009]