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## Homework Statement

Let k and n be positive integers. In how many ways are there integers a

_{1}≤ a

_{2}≤ ... ≤ a

_{k}≤ n.

## Homework Equations

## The Attempt at a Solution

I don't really know where to begin. Simply using permutations doesn't seem to work. I know that for a

_{1}, there are n integers to choose from. For the next number, there are 1 + (n-a

_{1}) integers to choose from. I'm reasonable sure that I can generalise this to say that for a

_{k}, there are 1+(n-a

_{k-1}) integers to choose from. From that point, I'm afraid I'm lost as to where to go with this.