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Exercise L

Posted 02/23/2015 and updated 03/01/2015
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`a(n)` is the number of lattice paths from `(0,0)` to `(n,2n)` that is not above the line `y=x`.

For example, a path from `(0,0)` to `(5,10)` not above `y=x` is illustrated in the following picture.

Find `a(n)`.


Updated 03/01/2015

Typos are fixed.