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I find the squares interesting - that the square of a number ending in 1 or 9 always ends in 1, 2 or 8 in 4, 3 or 7 in 9, 4 or 6 in 6, 0 in 0 and 5 in 5.
There are a lot of other cool properties, and, of course, squares are just special cases of multiplication in general.
Maybe we should have a math(s)-specific forum.
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