`\{P_n\}` is a sequence of posets and the Hasse diagrams of `P_1`, `P_2`, `P_3` and `P_4` are drawn in the following picture.
`a(n)` is the number of linear extensions of `P_n`.
Find `a(n)`.
`\{P_n\}` is a sequence of posets and the Hasse diagrams of `P_1`, `P_2`, `P_3` and `P_4` are drawn in the following picture.
Find `a(n)`.