The bestknown properties and formulas for the cosine function
Values in points
Students usually learn the following basic table of cosine function values for special points of the circle:
General characteristics
For real values of argument , the values of are real.
In the points , the values of are algebraic. In several cases they can even be rational numbers, 0, or 1. Here are some examples:
The values of can be expressed using only square roots if and is a product of a power of 2 and distinct Fermat primes {3, 5, 17, 257, …}.
The function is an entire analytical function of that is defined over the whole complex ‐plane and does not have branch cuts and branch points. It has an essential singular point at . It is a periodic function with the real period :
The function is an even function with mirror symmetry:
Differentiation
The derivatives of have simple representations using either the function or the function:
Ordinary differential equation
The function satisfies the simplest possible linear differential equation with constant coefficients:
The complete solution of this equation can be represented as a linear combination of and with arbitrary constant coefficients and :
The function also satisfies firstorder nonlinear differential equations:
Series representation
The function has a simple series expansion at the origin that converges in the whole complex ‐plane:
For real this series can be interpreted as the real part of the series expansion for the exponential function :
Product representation
The following famous infinite product representation for clearly illustrates that at :
Indefinite integration
Indefinite integrals of expressions involving the cosine function can sometimes be expressed using elementary functions. However, special functions are frequently needed to express the results even when the integrands have a simple form (if they can be evaluated in closed form). Here are some examples:
The last integral cannot be evaluated in closed form using the known classical special functions for arbitrary values of parameters and .
Definite integration
Definite integrals that contain the cosine function are sometimes simple. For example, the famous Dirichlet type and Fresnel integrals have the following values:
where is the Euler‐Mascheroni constant .
Some special functions can be used to evaluate more complicated definite integrals. For example, elliptic integrals and gamma functions are needed to express the following integrals:
Integral transforms
Integral transforms of expressions involving the cosine function may not be classically convergent but can be interpreted in a generalized functions (distributions) sense. For example, the exponential Fourier transform of the cosine function does not exist in the classical sense but can be expressed using the Dirac delta function.
Among other integral transforms of the cosine function, the best known are the Fourier cosine and sine transforms, and the Laplace, Mellin, Hilbert, and Hankel transforms:
Finite summation
The following finite sums from the cosine can be expressed using the trigonometric functions:
Infinite summation
The following infinite sums can be expressed using elementary functions:
Finite products
The following finite products from the cosine can be expressed using trigonometric functions:
Infinite products
The following infinite product that contains the cosine function can be expressed using the sine function:
Addition formulas
The cosine of a sum can be represented by the rule: "the cosine of a sum is equal to the product of the cosines minus the product of the sines." A similar rule is valid for the cosine of the difference:
Multiple arguments
In the case of multiple arguments , , , …, the function can be represented as the finite sum of terms that include powers of the sine and the cosine:
The function can also be represented as the finite sum including only the cosine of :
Halfangle formulas
The cosine of the half‐angle can be represented by the following simple formula that is valid in some vertical strips:
To make this formula correct for all complex , a complicated prefactor is needed:
where contains the unit step, real part, imaginary part, and the floor functions.
Sums of two direct functions
The sum of two cosine functions can be described by the rule: "the sum of the cosines is equal to two times the cosine of the half‐difference multiplied by the cosine of the half‐sum." A similar rule is valid for the difference of two cosines:
Products involving the direct function
The product of two cosine functions and the product of the cosine and sine have the following representations:
Powers of the direct function
The integer powers of the cosine functions can be expanded as finite sums of cosine functions with multiple arguments. These sums include binomial coefficients:
Inequalities
The bestknown inequalities for cosine functions are the following:
Relations with its inverse function
There are simple relations between the function and its inverse function :
The second formula is valid at least in the vertical strip . Outside of this strip, a much more complicated relation (that contains the unit step, real part, and the floor functions) holds:
Representations through other trigonometric functions
Cosine and sine functions are connected by a very simple formula including the linear function in the argument:
Another famous formula, connecting and , is shown in the well‐known Pythagorean theorem:
The last restriction on can be removed, but the formula will get a complicated coefficient with , that contains the unit step, real part, imaginary part, and the floor function:
The cosine function can also be represented using other trigonometric functions by the following formulas:
Representations through hyperbolic functions
The cosine function has representations using the hyperbolic functions:
Applications
The cosine function is used throughout mathematics, the exact sciences, and engineering.
