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I am trying to prove this statement, but am having some trouble with it. I think I am in the right direction but would like some feedback. Note: The proof must be completed using induction, and it looks like I need strong induction.

For every natural number n which is greater than or equal to 12, n can be written as the sum of a nonnegative multiple of 4 and a nonnegative multiple of 5.

Hint: in the inductive step, it is easiest to show that P(k − 3) → P(k + 1), where P(n) is the given proposition

So far, I have that n = 4a + 5b for some positive integer a,b. I did 4 base cases and verified that n = 12, 13, 14, 15 are true. I don't quite understand the hint. I tried a different way, and it seems to work but I'm not sure if it's right.

Base Case (n=12,13,14,15): Proven separately and fairly trivial.

Inductive Step: Suppose that the proposition is true for some n >= 12. Assume that for all natural numbers 12 <= f <= n, f = 4c + 5d for some integer c,d.

We want to show that n + 4 = 4a + 5b. Using our inductive hypothesis, we know that n <= n and thus n + 4 = (4c + 5d) + 4 = 4 (c + 1) + 5d. Since c is an integer, we know that c+1 is an integer as well and thus n + 4 = 4a + 5b (letting c+1 = a and d = b).

Since we have shown that P(n) ---> P(n+4), the proposition is true. That is, we have proven the proposition for n = 12, 16, 20,.... and n = 13, 17, 21, 25 and n = 14, 18, 22.... and n = 15, 19, 23, 27...

Minatorr
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2 Answers2

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Note that proving $P(n) \implies P(n+4)$ when $n \geq 12$ is the same thing as proving $P(k-3) \implies P(k+1)$ when $k \geq 15.$ Since you needed to include four statements of the proposition, $P(n)$ for $12 \leq n \leq 15,$ in your base case, your proof covers all the integers $n = 12$ or greater in the same way as the hint does.

The only caution I would have is that we do not usually define the natural numbers four at a time, that is, they are not axiomatically the numbers $0, 1, 2, 3$ and $n + 4$ for any $n$ that is a natural number, or $1,2,3,4$ and $n + 4$ for any $n$ that is a natural number, or anything else where the inductive part of the definition generates $n+4$ from $n.$ Instead, we get $n+1$ from $n.$

So I'm not sure exactly how your instructor would prefer to word it, but the hint suggests to me that instead of taking $P(n)$ as your proposition, where $P(n)$ means $n=4a+5b$ for non-negative integers $a,b,$ you should instead take $P'(n) = P(n)\land P(n-1) \land P(n-2) \land P(n-3)$ as your proposition, with base case $n=15.$ Then in order to prove that $P'(k) \implies P'(k+1),$ you use the fact that $P(k - 3) \implies P(k+1)$ and the fact that each of the other three sub-propositions implies itself.

On the other hand, if a statement $P(n)$ is true for some set of $m$ consecutive integers $n_0, n_0 + 1, \ldots, n_0 + m - 1,$ and if $P(n) \implies P(n+m),$ then $P(n)$ is true for all integers $n$ such that $n \geq n_0.$ Given this general fact about inductive proofs, your proof shows what it was desired to show.

David K
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$$12+4k=(3+k)\times4$$ $$13+4k=(2+k)\times4+1\times5$$ $$14+4k=(1+k)\times4+2\times5$$ $$15+4k=k\times4+3\times5$$ Since every number $n\ge12$ can be written one of the above forms where $k\in\mathbb{N}$, we can write any number $n\ge12$ in the desired form.

Peter Foreman
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