Av(12, 321)

length 2: 21

Generating function

$A(x) = x^{2} + x + 1$

Coefficients

1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, ...

System of equations

$\operatorname{F_{2}}{\left (x \right )} = \operatorname{F_{0}}{\left (x \right )} + \operatorname{F_{1}}{\left (x \right )}$

$\operatorname{F_{0}}{\left (x \right )} = 1$

$\operatorname{F_{1}}{\left (x \right )} = x \left(x + 1\right)$