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Hyperbolic functions

Inmathematics, hyperbolic functions are analogues of the ordinary trigonometric functions, but defined using the hyperbola rather than the circle. Just as the points (cos t, sin t) form a circle with a unit radius, the points (cosh t, sinh t) form the right half of the unit hyperbola. Also, similarly to how the derivatives of sin(t) and cos(t) are cos(t) and –sin(t) respectively, the derivatives of sinh(t) and cosh(t) are cosh(t) and +sinh(t) respectively.

Hyperbolic functions occur in the calculations of angles and distances in hyperbolic geometry. They also occur in the solutions of many linear differential equations (such as the equation defining a catenary), cubic equations, and Laplace's equationinCartesian coordinates. Laplace's equations are important in many areas of physics, including electromagnetic theory, heat transfer, fluid dynamics, and special relativity.

The basic hyperbolic functions are:^[1]

• hyperbolic sine "sinh" (/ˈsɪŋ, ˈsɪntʃ, ˈʃaɪn/),^[2]
• hyperbolic cosine "cosh" (/ˈkɒʃ, ˈkoʊʃ/),^[3]

from which are derived:^[4]

• hyperbolic tangent "tanh" (/ˈtæŋ, ˈtæntʃ, ˈθæn/),^[5]
• hyperbolic cotangent "coth" (/ˈkɒθ, ˈkoʊθ/),^[6][7]
• hyperbolic secant "sech" (/ˈsɛtʃ, ˈʃɛk/),^[8]
• hyperbolic cosecant "csch" or "cosech" (/ˈkoʊsɛtʃ, ˈkoʊʃɛk/^[3])

corresponding to the derived trigonometric functions.

The inverse hyperbolic functions are:

• area hyperbolic sine "arsinh" (also denoted "sinh⁻¹", "asinh" or sometimes "arcsinh")^[9][10][11]
• area hyperbolic cosine "arcosh" (also denoted "cosh⁻¹", "acosh" or sometimes "arccosh")
• area hyperbolic tangent "artanh" (also denoted "tanh⁻¹", "atanh" or sometimes "arctanh")
• area hyperbolic cotangent "arcoth" (also denoted "coth⁻¹", "acoth" or sometimes "arccoth")
• area hyperbolic secant "arsech" (also denoted "sech⁻¹", "asech" or sometimes "arcsech")
• area hyperbolic cosecant "arcsch" (also denoted "arcosech", "csch⁻¹", "cosech⁻¹","acsch", "acosech", or sometimes "arccsch" or "arccosech")

The hyperbolic functions take a real argument called a hyperbolic angle. The size of a hyperbolic angle is twice the area of its hyperbolic sector. The hyperbolic functions may be defined in terms of the legs of a right triangle covering this sector.

Incomplex analysis, the hyperbolic functions arise when applying the ordinary sine and cosine functions to an imaginary angle. The hyperbolic sine and the hyperbolic cosine are entire functions. As a result, the other hyperbolic functions are meromorphic in the whole complex plane.

ByLindemann–Weierstrass theorem, the hyperbolic functions have a transcendental value for every non-zero algebraic value of the argument.^[12]

Hyperbolic functions were introduced in the 1760s independently by Vincenzo Riccati and Johann Heinrich Lambert.^[13] Riccati used Sc. and Cc. (sinus/cosinus circulare) to refer to circular functions and Sh. and Ch. (sinus/cosinus hyperbolico) to refer to hyperbolic functions. Lambert adopted the names, but altered the abbreviations to those used today.^[14] The abbreviations sh, ch, th, cth are also currently used, depending on personal preference.

Notation[edit]

Definitions[edit]

There are various equivalent ways to define the hyperbolic functions.

Exponential definitions[edit]

In terms of the exponential function:^[1][4]

• Hyperbolic sine: the odd part of the exponential function, that is,
  $${\displaystyle \sinh x={\frac {e^{x}-e^{-x}}{2}}={\frac {e^{2x}-1}{2e^{x}}}={\frac {1-e^{-2x}}{2e^{-x}}}.}$$
  
• Hyperbolic cosine: the even part of the exponential function, that is,
  $${\displaystyle \cosh x={\frac {e^{x}+e^{-x}}{2}}={\frac {e^{2x}+1}{2e^{x}}}={\frac {1+e^{-2x}}{2e^{-x}}}.}$$
  
• Hyperbolic tangent:
  $${\displaystyle \tanh x={\frac {\sinh x}{\cosh x}}={\frac {e^{x}-e^{-x}}{e^{x}+e^{-x}}}={\frac {e^{2x}-1}{e^{2x}+1}}.}$$
  
• Hyperbolic cotangent: for x ≠ 0,
  $${\displaystyle \coth x={\frac {\cosh x}{\sinh x}}={\frac {e^{x}+e^{-x}}{e^{x}-e^{-x}}}={\frac {e^{2x}+1}{e^{2x}-1}}.}$$
  
• Hyperbolic secant:
  $${\displaystyle \operatorname {sech} x={\frac {1}{\cosh x}}={\frac {2}{e^{x}+e^{-x}}}={\frac {2e^{x}}{e^{2x}+1}}.}$$
  
• Hyperbolic cosecant: for x ≠ 0,
  $${\displaystyle \operatorname {csch} x={\frac {1}{\sinh x}}={\frac {2}{e^{x}-e^{-x}}}={\frac {2e^{x}}{e^{2x}-1}}.}$$
  

Differential equation definitions[edit]

The hyperbolic functions may be defined as solutions of differential equations: The hyperbolic sine and cosine are the solution (s, c) of the system

${\displaystyle s(0)=0,c(0)=1.}$

${\displaystyle (ae^{x}+be^{-x},ae^{x}-be^{-x})}$

sinh(x) and cosh(x) are also the unique solution of the equation f ″(x) = f (x), such that f (0) = 1, f ′(0) = 0 for the hyperbolic cosine, and f (0) = 0, f ′(0) = 1 for the hyperbolic sine.

Complex trigonometric definitions[edit]

Hyperbolic functions may also be deduced from trigonometric functions with complex arguments:

• Hyperbolic sine:^[1]
  $${\displaystyle \sinh x=-i\sin($$ix).}
  
• Hyperbolic cosine:^[1]
  $${\displaystyle \cosh x=\cos($$ix).}
  
• Hyperbolic tangent:
  $${\displaystyle \tanh x=-i\tan($$ix).}
  
• Hyperbolic cotangent:
  $${\displaystyle \coth x=i\cot($$ix).}
  
• Hyperbolic secant:
  $${\displaystyle \operatorname {sech} x=\sec($$ix).}
  
• Hyperbolic cosecant:
  $${\displaystyle \operatorname {csch} x=i\csc($$ix).}
  

where i is the imaginary unit with i² = −1.

The above definitions are related to the exponential definitions via Euler's formula (See § Hyperbolic functions for complex numbers below).

Characterizing properties[edit]

Hyperbolic cosine[edit]

It can be shown that the area under the curve of the hyperbolic cosine (over a finite interval) is always equal to the arc length corresponding to that interval:^[15]

Hyperbolic tangent[edit]

The hyperbolic tangent is the (unique) solution to the differential equation f ′ = 1 − f ², with f (0) = 0.^[16][17]

Useful relations[edit]

The hyperbolic functions satisfy many identities, all of them similar in form to the trigonometric identities. In fact, Osborn's rule^[18] states that one can convert any trigonometric identity (up to but not including sinhs or implied sinhs of 4th degree) for ${\displaystyle \theta }$, ${\displaystyle 2\theta }$, ${\displaystyle 3\theta }$or${\displaystyle \theta }$ and ${\displaystyle \varphi }$ into a hyperbolic identity, by expanding it completely in terms of integral powers of sines and cosines, changing sine to sinh and cosine to cosh, and switching the sign of every term containing a product of two sinhs.

Odd and even functions:

Hence:

Thus, cosh x and sech x are even functions; the others are odd functions.

Hyperbolic sine and cosine satisfy:

the last of which is similar to the Pythagorean trigonometric identity.

One also has

for the other functions.

Sums of arguments[edit]

Also:

Subtraction formulas[edit]

Also:^[19]

Half argument formulas[edit]

where sgn is the sign function.

Ifx ≠ 0, then^[20]

Square formulas[edit]

Inequalities[edit]

The following inequality is useful in statistics: ${\displaystyle \operatorname {cosh} ($t)\leq e^{t^{2}/2}} ^[21]

It can be proved by comparing term by term the Taylor series of the two functions.

Inverse functions as logarithms[edit]

Derivatives[edit]

Second derivatives[edit]

Each of the functions sinh and cosh is equal to its second derivative, that is:

All functions with this property are linear combinationsofsinh and cosh, in particular the exponential functions ${\displaystyle e^{x}}$ and ${\displaystyle e^{-x}}$.^[22]

Standard integrals[edit]

The following integrals can be proved using hyperbolic substitution:

where C is the constant of integration.

Taylor series expressions[edit]

It is possible to express explicitly the Taylor series at zero (or the Laurent series, if the function is not defined at zero) of the above functions.

The sum of the sinh and cosh series is the infinite series expression of the exponential function.

The following series are followed by a description of a subset of their domain of convergence, where the series is convergent and its sum equals the function.