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  "body": "\n## Summary\n\nAn involution is a function from a set to itself that undoes itself: applying it twice gives back the starting point.\nEquivalently, it is a bijection that is its own [[inverse|0J2WAR83WP]].\nNegation of numbers, [[complex conjugation|0FTH2E9TAS]], the [[complement|0RKF1ZT5JJ]] of a set and the inverse map of a [[group|G5S1M9]] are involutions.\nIn algebra the word is also used for an involution that reverses a product, $(xy)^* = y^* x^*$, as in rings with involution.\n\n## Explanations\n\n### L1 human\n\nAn involution is an action that, done twice, puts everything back.\nReflecting a picture in a mirror twice gives the original picture.\nChanging the sign of a number twice gives the original number: $-(-5) = 5$.\nAdding $1$ is not an involution, because doing it twice adds $2$.\n",
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