Brett Fazio's Competitive Programming Hackpack, last updated, migrated to BitBucket.
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Pascal's triange
Fibonacci properties
Catalan number uses (http://oldweb.sbc.edu/sites/default/files/Honors/XiaotongJiang.July20_0.pdf)
Modulo arithmetic rules
Geometry formula
Law of Tangents
Pancake sorting
Max 2D range sum
traveling salesman
roman numerals
knight moves
https://owlcation.com/stem/Circle-Inscribed-in-Square-Geometry-Problems-Solutions