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# Inverse hyperbolic trigonometric functions

The inverses of the hyperbolic trigonometric functions (hyperbolic functions) are the area hyperbolic functions. The names hint at the fact that they give the area of a sector of the unit hyperbola x 2y 2 = 1 in the same way that the inverse circular trigonometric functions (inverse trigonometric functions) give the length of an arc of the unit circle x 2 + y 2 = 1.

The abbreviations arcsinh, arccosh, etc., are commonly used, even though they are misnomers, since the prefix arc is the abbreviation for arcus, while the prefix ar stands for area.[1] [2] [3]

In computer science this is often shortened to asinh, acosh, etc. The notation sinh −1(x), cosh −1(x), etc., are also used, despite the fact that care must be taken to avoid misinterpretations of the superscript −1 as a power as opposed to a shorthand for inverse (e.g., cosh −1(x) versus cosh(x) −1).

The values of area hyperbolic functions (inverse hyperbolic functions) are hyperbolic areas (area of a sector of the unit hyperbola).

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## Notes

1. As stated by Jan Gullberg, Mathematics: From the Birth of Numbers (New York: W. W. Norton & Company, 1997), ISBN 039304002X, p. 539:

Another form of notation, arcsinh x, arccosh x, etc., is a practice to be condemned as these functions have nothing whatever to do with arc, but with area, as is demonstrated by their full Latin names,

arsinh     area sinus hyperbolicus
arcosh     area cosinus hyperbolicus, etc.

2. As stated by Eberhard Zeidler, Wolfgang Hackbusch and Hans Rudolf Schwarz, translated by Bruce Hunt, Oxford Users' Guide to Mathematics (Oxford: Oxford University Press, 2004), ISBN 0198507631, Section 0.2.13: "The inverse hyperbolic functions", p. 68: "The Latin names for the inverse hyperbolic functions are area sinus hyperbolicus, area cosinus hyperbolicus, area tangens hyperbolicus and area cotangens hyperbolicus (of x). ..." This aforesaid reference uses the notations arsinh, arcosh, artanh, and arcoth for the respective inverse hyperbolic functions.
3. As stated by Ilja N. Bronshtein, Konstantin A. Semendyayev, Gerhard Musiol and Heiner Muehlig, Handbook of Mathematics (Berlin: Springer-Verlag, 5th ed., 2007), ISBN 3540721215, doi:10.1007/978-3-540-72122-2, Section 2.10: "Area Functions", p. 91:

The area functions are the inverse functions of the hyperbolic functions, i.e., the inverse hyperbolic functions. The functions sinh x, tanh x, and coth x are strictly monotone, so they have unique inverses without any restriction; the function cosh x has two monotonic intervals so we can consider two inverse functions. The name area refers to the fact that the geometric definition of the functions is the area of certain hyperbolic sectors (...)