Although I did a lot of my functional-analysis-definitions for 2015 I certainly didn’t let that pass from the stage. My letter-I choice for the End 2016 A-to-Z was Image. This is useful when you think about functions as ways to turn one set (the domain) into another set (the range). A lot of definitions and tests for conditions, such as continuity, become much less technical and fussy when you start looking at images. It’s one of those changes in perspective that makes work easier.
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