length 2: 21
length 3: 321
length 4: 4321
length 5: 54321
$\operatorname{F_{2}}{\left (x \right )} = \operatorname{F_{0}}{\left (x \right )} + \operatorname{F_{1}}{\left (x \right )}$
$\operatorname{F_{0}}{\left (x \right )} = 1$
$\operatorname{F_{1}}{\left (x \right )} = x \operatorname{F_{2}}{\left (x \right )}$