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I've got the following homework question. If anybody could possibly point me in the right direction, that would be great:

Suppose X is a sequence with 10 terms built from 26 letters {a, b, c, ..., z} such that e, n, r, and s occur at least once. Please determine with proof the number of such Xs.

I'm looking to create an exponential generating function to solve this problem. From what I understand, if ALL 26 letters had to occur at least once, the exponential generating function could be described as $ (e^x-1)^{26}$.

Since there are $4$ values that must occur at least once, my assumption is that the generating function will be $$(e^x-1)^4(e^x)^{22}$$

And then to find the actual answer, I believe I will need the coefficient on $$\frac{x^{10}}{10!}$$

Am I on the right track here?

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You are right. Now multiply out and get the coefficients.

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  • $\begingroup$ Alright, thanks! I've had a bit of trouble grasping the concept of EGF's, so confirmation that I'm doing this right is really helpful. $\endgroup$ – Nate Apr 20 '14 at 0:15
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    $\begingroup$ Take a peek at analytic combinatorics, or at Segdewick and Flajolet's "Introduction to the Analysis of Algorithms", check out the basic proofs for labelled structures. I believe this will clarify the mess (and give you tools to solve all sorts of hairy problems in a few lines). $\endgroup$ – vonbrand Apr 20 '14 at 0:20

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