### Av(12, 21)

#### Generating function

$A(x) = x + 1$

#### Coefficients

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

#### System of equations

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

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

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