Problem solving

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George Pólya’s "How To Solve It" (which is discussed, briefly, in the "solving problems" section of this document)

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Paul Zeitz’s "The Art and Craft of Problem Solving"

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"Thinking Mathematically" by John Mason et al. (https://www.goodreads.com/book/show/1241663.Thinking_Mathematically)

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Darij Grinberg’s \sayMathematical Problem Solving (freely available on the internet at https://www.cip.ifi.lmu.de/~grinberg/t/20f/mps.pdf)