Define f: (N U {0}) -> N by Prove that for all non-negative numbers, n, f(n) is divisible by 9.

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Ok so for this one P(n) is going to be the statement "f(n) is divisible by 9". P(0) holds and we can now assume that this holds true for all j<k.

This part of induction always gets me, now I have to show P(j) -> P(k), or P(n) -> P(n+1).

If 9 divides f(j), then there is some number, D, such that . Now I know there's some way to manipulate this to get what I need but I don't see it.

Hints?