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Many infinite sets appear in mathematics: the set of counting numbers; the set of integers; the set of rational numbers; the set of irrational numbers; the set of real numbers; the set of complex numbers. Also, certain subsets of these, such as the set of square numbers, the set of prime numbers, and others.
using contraction and expansion
The validity or invalidity of a function are not abstract but depend on its domain and codomain or range. If for any point, A, in the domain there is a unique point, B, in the range such that f(A) = B then the function is valid at A. The validity of a function can change from point to point. For example, f(x) = sqrt(x) is not a function from the set of Real Numbers to the set of Real Numbers because any negative number in the domain is not mapped to any value in the range. This can be corrected either by changing the domain to the set of non-negative Real Numbers or (if you are a more advanced mathematician) change the range to the set of Complex Numbers. Similarly the reciprocal function, f(x) = 1/x is valid everywhere except for x = 0. Or f(x) = tan(x) is valid except for x = 90+k*180 degrees for all integer values of k - so it is not valid at an infinite number of points.
In a certain sense, the set of complex numbers is "larger" than the set of real numbers, since the set of real numbers is a proper subset of it.
All of the natural numbers.