Posted 02/24/2015 and updated 06/28/2016

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This problem is proposed by**Ran Pan**. If you want to submit your problem, please click here.

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This problem is proposed by

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$a(n)$ is the number of linear extensions of poset $P_n$.

One can figure out what the Hasse diagram of $P_n$ is by observing Hasse diagrams of $P_1$, $P_2$, $P_3$, $P_4$ and $P_5$ in the picture.

Find $a(n)$.

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Updated 06/11/2015 and remarked by **Ran Pan**

Updated 06/28/2016 and remarked by **Ran Pan**

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