Miscellaneous

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BlackPenRedPen, a YouTube channel (https://www.youtube.com/channel/UC_SvYP0k05UKiJ_2ndB02IA)

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Stephen Miller’s "The Probability Lifesaver"  a wonderful book on probability

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Jiří Lebl’s "Notes on Diffy Qs" (https://www.jirka.org/diffyqs/diffyqs.pdf)

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The American Institute of Mathematics maintain a list of open access textbooks of which they approve (https://aimath.org/textbooks/approvedtextbooks/).

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Ian Anderson’s "A First Course in Discrete Mathematics"

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Philip J. Davis and Reuben Hersh’s "The Mathematical Experience"

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I found Alan Schoenfeld’s "Mathematical Thinking and Problem Solving" interesting, but I can see why many other people might not.

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Michael Steele’s \sayThe CauchySchwarz Master Class.