OFFSET
0,4
LINKS
Index entries for linear recurrences with constant coefficients, signature (6,-12,10,-5,1).
FORMULA
G.f.: x^2*(x-1)/((x^2-3*x+1)*(x^3-2*x^2+3*x-1)). - Alois P. Heinz, Sep 15 2020
EXAMPLE
a(0) = a(1) = 0, because no word of length n < 2 can contain 12.
a(2) = 1, because there is one word of length 2 and it is 12.
a(3) = 5, because there are 5 words of length 3 and they are 121, 112, 212, 122, 120.
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Mauricio J. Santos, Sep 15 2020
EXTENSIONS
a(20)-a(29) from Alois P. Heinz, Sep 15 2020
STATUS
approved