# Induction on a Sum of Natural Numbers

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Let f:N x N -> N be the function defined recursively as follows:

f(0, 0) = 6

f(i, j) = f(i - 1, j) + 2 if i > 0 and j = 0

f(i, j) = f(i, j - 1) + 1 if j > 0

Use induction on the sum i + j to prove that f(i, j) = 2i + j + 6 for all (i, j) in N x N.

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Since i >= 0 and j >= 0, we have that i + j >= 0.

Base case: i + j = 0

In this case, (i, j) = (0, 0). By definition, f(0, 0) = 6, so

f(i, j) = f(0, 0) = 6 = 0 + 0 + 6 = 2(0) + 0 + 6 = 2i + j + 6

Induction step: Let k >= 1, let i, ...

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