Inverse hyperbolic function
In mathematics, the inverse hyperbolic functions provide a hyperbolic angle corresponding to a given value of a hyperbolic function. The size of the hyperbolic angle is equal to the area of the corresponding hyperbolic sector of the hyperbola xy = 1, or twice the area of the corresponding sector of the unit hyperbola x2 − y2 = 1, just as a circular angle is twice the area of the circular sector of the unit circle. Some authors have called inverse hyperbolic functions "area functions" to realize the hyperbolic angles.
The preferred abbreviations are ar- followed by the hyperbolic function (arsinh, arcosh, etc.).
However, arc- followed by the hyperbolic function (for example arcsinh, arccosh), are also commonly seen by analogy with the nomenclature for inverse trigonometric functions. The latter are misnomers, since the prefix arc is the abbreviation for arcus, while the prefix ar stands for area. 
Other authors prefer to use the notation argsinh, argcosh, argtanh, and so on, where the prefix arg is the abbreviation of the Latin argumentum. In computer science this is often shortened to asinh.
The notation sinh−1(x), cosh−1(x), etc., is 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 operators are defined in the complex plane by:
The above square roots are principal square roots, and the are principal complex logarithms. For real arguments, i.e., z = x, which return real values, certain simplifications can be made e.g. and , which are not generally true for complex arguments.
Expansion series can be obtained for the above functions:
Asymptotic expansion for the arsinh x is given by
For an example differentiation: let θ = arsinh x, so (where sinh2 θ = (sinh θ)2):
Composition of hyperbolic and inverse hyperbolic functions
- As stated by Jan Gullberg, Mathematics: From the Birth of Numbers (New York: W. W. Norton & Company, 1997), ISBN 0-393-04002-X, 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.
- 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 0-19-850763-1, 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.
- As stated by Ilja N. Bronshtein, Konstantin A. Semendyayev, Gerhard Musiol and Heiner Muehlig, Handbook of Mathematics (Berlin: Springer-Verlag, 5th ed., 2007), ISBN 3-540-72121-5, 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 ...
- Bacon, Harold Maile (1942). Differential and Integral Calculus. McGraw-Hill. p. 203.
- Press, WH; Teukolsky, SA; Vetterling, WT; Flannery, BP (1992). "Section 5.6. Quadratic and Cubic Equations". Numerical Recipes in FORTRAN: The Art of Scientific Computing (2nd ed.). New York: Cambridge University Press. ISBN 0-521-43064-X.
- Herbert Busemann and Paul J. Kelly (1953) Projective Geometry and Projective Metrics, page 207, Academic Press.