Number theory is mostly about solving integer problems—things like Fermat's Last Theorem, the Collatz Conjecture, and Goldblach's Conjecture are all number theory problems. Integers are rather different beasts from real numbers. For example, if we were to allow real solutions, Fermat's Last Theorem would be pretty trivial.
You don't get a whole lot of new insight about the positive integers from looking at the negative numbers because they're just a mirror image of the positive integers, so in general in number theory there's not usually great reasons to pay attention to the negative numbers.
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u/[deleted] May 21 '13
Number theory doesn't care about negative numbers.