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Substantiivi

1. (set theory) A function for which every element of the codomain is mapped to by some element of the domain; (formally) Any function ${\displaystyle f:X\rightarrow Y}$ for which for every ${\displaystyle y\in Y}$, there is at least one ${\displaystyle x\in X}$ such that ${\displaystyle f(x)=y}$.