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The same notion can be defined using - recursion:

rank(x) = sup{rank(y) + 1 : y ∈ x} , assuming sup{∅} = 0. rational function A function that is expressible as a quotient of polynomials.

rational number A real number that can be expressed as a quotient of integers. Furthermore, the digits of the decimal expansion of a real number will consist of a sequence which,

eventually, repeats periodically if and only if the number is a rational number. The set of all rational numbers is normally denoted Q or Q.