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Nov 24, 2020 at 14:28 comment added antkam A $13$-card s-f can be represented as a pair-of-sets $(X,Y)$ by picking any of the $5$ sub-sequences as $X$ and the rest as $Y$. Since $5 \times {43 \choose 4}$ counts such pairs-of-sets, any hand with a $13$-long s-f contributes a count of $5$ towards that total, hence $5 N_{13}$.
Nov 24, 2020 at 9:29 comment added true blue anil I think you have written the coefficients in reverse order. How can there be multiple ways of representing a $13$ card s-f ?
Nov 24, 2020 at 6:17 history edited antkam CC BY-SA 4.0
deleted 13 characters in body
Nov 24, 2020 at 6:11 history answered antkam CC BY-SA 4.0