题目描述已知一个数列 a，它满足：ax={1x∈{1,2,3}ax−1+ax−3x≥4a_x= \begin{cases} 1 & x \in\{1,2,3\}\\ a_{x-1}+a_{x-3} & x \geq 4 \end{cases}ax={1ax−1+ax−3x∈{
