Partial Fractions May Help Here!

Calculus Level 4

The expression $$\displaystyle \sum_{n=a}^{\infty} \sum_{b=n}^{\infty} \frac{1}{(n-a)!(b-n)!}$$ , for $$a,b,n \in N$$, can be expressed as $$k^2$$. Find $$k$$.

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