What is the difference between one-one (injective) and onto (surjective)?
One-one means different inputs give different outputs (no output repeats); onto means every element of the codomain is actually hit. A function needs both (bijective) to be invertible.
One-one means different inputs give different outputs (no output repeats); onto means every element of the codomain is actually hit. A function needs both (bijective) to be invertible. A reliable way to keep them apart is to focus on what each one is defined by, not just the similar-sounding words.
Key point
one-one (injective) vs onto (surjective)
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