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### Number theory

The number theory originally studied primarily the properties of integers.

##### Explanation

There is still a lot not known about numbers, and not even about whole numbers.

##### Example 1

Goldbach's conjecture says about even numbers, that they would consist of the sum of two prime numbers, as with

10 = 3 + 7 = 5 + 5

We don't know if that's always the case.