it is called bijective or a one-to-one correspondence. Functions are not limited to those one can draw an arrow diagram for, so long as the function is well-defined. That is, for each possible input, the output is defined. For example, one may define a function on the natural numbers by multiplying by two:

Etymology and related terms

The term cardinality originates from the post-classical Latin cardo ("to hinge"), which referred to

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