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That is, if a function's arity is the number of inputs it has, its __ is the number of outputs it has. (Fill in the blank.)

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    One. That is what you call the number of outputs a function has. It is sometimes written in symbols as 1. :)2012-08-02
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    In the usual definition, of course, functions have a single input and a single output, but if all elements of the domain are $k$-tuples for the same value of $k$, you might say you have a "function of $k$ variables" or a "$k$-ary operation" (though I don't think people usually use the words "arity" and "function" together, prefering to call functions "operations" when talking about arities), and you might want a term for the dual case, but I haven't heard any such name (if I had to make one up I'd say co-arity). It's not as popular an idea somehow, e.g., people prefer operads to PROPs.2012-08-02

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