Plural of factorial; mathematical quantities obtained by multiplying a number by all positive whole numbers below it (written with an exclamation mark, like 5! = 5×4×3×2×1).
From 'factorial' (relating to factors), which comes from Latin 'factor' (maker, doer). The symbol '!' was chosen in the 1800s to give this operation a distinctive mark.
The factorial function is nature's counter for 'how many ways can we arrange this?'—it appears in genetics, physics, statistics, and probability. It's simple but sneakily important across all of science.
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