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It seems like a typo where "integers" is used when the intention was to write "natural numbers". That is the solution to exercise 194 part a) which asked if the set of natural numbers is a field.

Whether 0 is a natural number is still fairly ambiguous; I remember being taught (1990s UK) to be specific about which definition was being used, or to prefer another name such as 'positive integers' or 'non-negative integers'

notation has unified dramatically since the 1960s.