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1: https://sites.lsa.umich.edu/hderksen/wp-content/uploads/sites/614/2018/05/ProblemSolving-1.pdf
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2: https://artofproblemsolving.com/articles/files/SatoNT.pdf
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3: https://math.charlotte.edu/sites/math.charlotte.edu/files/fields/preprint_archive/paper/2007_09_0.pdf
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4: https://www.maa.org/sites/default/files/pdf/Putnam/Competition_Archive/2017PutnamProblemsSolutions.pdf
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5: https://www.math.ucla.edu/~mt/131a.1.02s/131A-HW-Sol.pdf
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6: https://www.math.uci.edu/~krubin/194/SolutionsA1B1.pdf
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7: http://www-groups.mcs.st-andrews.ac.uk/~john/MISS.pdf
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8: http://math.northwestern.edu/~mlerma/problem_solving/putnam/training_problems-2020.pdf
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