My 2015 A-to-Z was before I asked for topic nominations. So what topics I did cover tended to focus on my own particular interests, and the courses I remember liking. So there was a good bit of functional analysis, or things that go into functional analysis. Into covers one of those topics. And you get some bonus coverage of “onto”. The terms describe ways that a function can use its range. To an extent it’s taxonomy, distinguishing functions that use the whole range versus those that use part. But it gives you vocabulary to discuss what cases you have to cover in proving a thing about a function. So it’s one of those small but useful terms.
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