## Math

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Math
> Number theory
> Natural Numbers

### Natural Numbers

Properties of the set of positive integers.#### Sibling topics:

#### Contents:

Definition of prime

For `t in NN` where `t>1`, `t` is **prime**if the only divisors of `t` are 1 and `t` itself. More concisely, if `t` is prime, then:

`(AA x in NN)(x div t => [x=1 or x=t])`

If `t` is not prime (and `t>1`), then `t` is **composite**. That is:

`(EE u,v in NN)(u>1 and v>1 and t=uv)`

The number 1 is neither prime nor composite.
**Theorem:**Product of natural number factors is not less

For all `a,b in NN`, `ab>=a and ab>=b`. That is, the product of two natural numbers is at least as large
as both of its factors.

**We'll first prove that `ab>=a`. Because `b` is a natural number, we know that `b>=1`. This gives two cases:**

*Proof:*- `b=1`. Since `n xx 1=n` for all `n`, we know that `ab=a`.
- `b>1`. Recall that if `x>y and c>0`, then `cx > cy`. If `x=b`, `y=1`, `c=a`, we have `b>1` and `a>0` (because `a` is a natural number), so `ab>a`.