What Are Hyperbolic Functions?
Hyperbolic functions are analogs of the ordinary trigonometric (circular) functions, but defined using the hyperbola rather than the circle. Just as trigonometric functions parameterize a unit circle (x2 + y2 = 1), the hyperbolic functions parameterize a unit hyperbola (x2 - y2 = 1).
They arise naturally in many areas of mathematics, including calculus, differential equations, complex analysis, and physics. They are defined in terms of the exponential function e^x.
The Six Hyperbolic Functions
sinh(x) - Hyperbolic Sine
The odd part of the exponential function. Domain: all reals. Range: all reals.
cosh(x) - Hyperbolic Cosine
The even part of the exponential function. Domain: all reals. Range: [1, infinity).
tanh(x) - Hyperbolic Tangent
Ratio of sinh to cosh. Domain: all reals. Range: (-1, 1).
csch(x) - Hyperbolic Cosecant
Reciprocal of sinh. Domain: all reals except 0. Range: all reals except 0.
sech(x) - Hyperbolic Secant
Reciprocal of cosh. Domain: all reals. Range: (0, 1].
coth(x) - Hyperbolic Cotangent
Reciprocal of tanh. Domain: all reals except 0. Range: (-inf, -1) U (1, inf).
Key Identities
Fundamental Identity
The hyperbolic analog of the Pythagorean identity sin2(x) + cos2(x) = 1:
cosh2(x) - sinh2(x) = 1
Note the minus sign, which contrasts with the plus sign in the circular case.
Other Important Identities
- sinh(-x) = -sinh(x) (odd function)
- cosh(-x) = cosh(x) (even function)
- sinh(x + y) = sinh(x)cosh(y) + cosh(x)sinh(y)
- cosh(x + y) = cosh(x)cosh(y) + sinh(x)sinh(y)
- e^x = cosh(x) + sinh(x)
- e^(-x) = cosh(x) - sinh(x)
- 1 - tanh2(x) = sech2(x)
- coth2(x) - 1 = csch2(x)
Derivatives
- d/dx sinh(x) = cosh(x)
- d/dx cosh(x) = sinh(x)
- d/dx tanh(x) = sech2(x)
- d/dx csch(x) = -csch(x) coth(x)
- d/dx sech(x) = -sech(x) tanh(x)
- d/dx coth(x) = -csch2(x)
Applications
- Catenary curves: A hanging chain or cable forms a catenary described by y = a cosh(x/a)
- Special relativity: The rapidity parameter uses hyperbolic functions for Lorentz boosts
- Engineering: Transmission line analysis, heat transfer, and signal processing
- Machine learning: tanh is used as an activation function in neural networks
- Complex analysis: Connecting trigonometric and hyperbolic functions via Euler's formula
Connection to Trigonometric Functions
Through complex numbers, the hyperbolic and trigonometric functions are intimately related:
- sinh(ix) = i sin(x)
- cosh(ix) = cos(x)
- sin(ix) = i sinh(x)
- cos(ix) = cosh(x)