We introduce the double factorial !!, also called the alternate factorial. This cute little bit of mathematical notation gives us a way to achieve an effect similar to the standard factorial but while skipping every other number. The double exclamation point will respect the parity of its number, and proceed accordingly. We'll see examples of the factorial and double factorial, and then discuss perfect matchings in graph theory to see an example of where the double factorial is the solution to a problem. #maths #graphtheory #mathematics

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