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=== Converting predicates to numeric functions ===
In some settings it is natural to consider primitive recursive functions that take as inputs tuples that mix numbers with [[truth value]]s (that is <math>t</math> for true and <math>f</math> for false),{{Citation needed|date=January 2025 |reason=Kleene never considers mixed domains - see p.226 where he lists the 4 types of functions he considers. Using N to represent the naturals, and T to represent the truth values: (a) N to N (b) N to T (c) T oto T (d) T to N}} or that produce truth values as outputs.<ref>Kleene [1952 pp. 226–227]</ref> This can be accomplished by identifying the truth values with numbers in any fixed manner. For example, it is common to identify the truth value <math>t</math> with the number <math>1</math> and the truth value <math>f</math> with the number <math>0</math>. Once this identification has been made, the [[indicator function|characteristic function]] of a set <math>A</math>, which always returns <math>1</math> or <math>0</math>, can be viewed as a predicate that tells whether a number is in the set <math>A</math>. Such an identification of predicates with numeric functions will be assumed for the remainder of this article.
=== Predicate "Is zero" ===
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