Closed-form expression

In mathematics, a closed form expression or formula is one that is formed with constants, variables, and a set of functions considered as basic and connected by arithmetic operations (+, −, ×, /, and integer powers) and function composition. Commonly, the basic functions that are allowed in closed forms are nth root, exponential function, logarithm, and trigonometric functions. However, the set of basic functions depends on the context. For example, if one adds polynomial roots to the basic functions, the functions that have a closed form are called elementary functions.

The closed-form problem arises when new ways are introduced for specifying mathematical objects, such as limits, series, and integrals: given an object specified with such tools, a natural problem is to find, if possible, a closed-form expression of this object; that is, an expression of this object in terms of previous ways of specifying it.

Example: roots of polynomials

The quadratic formula

x

=

−

b

±

b

2

−

4

a

c

2

a

{\displaystyle x={\frac {-b\pm {\sqrt {b^{2}-4ac}}}{2a}}}

is a closed form of the solutions to the general quadratic equation

a

x

2

+

b

x

+

c

=

0.

{\displaystyle ax^{2}+bx+c=0.}

More generally, in the context of polynomial equations, a closed form of a solution is a solution in radicals; that is, a closed-form expression for which the allowed functions are only nth-roots and field operations

(

+

,

−

,

×

,

/

)

.

{\displaystyle (+,-,\times ,/).}

In fact, field theory allows showing that if a solution of a polynomial equation has a closed form involving exponentials, logarithms or trigonometric functions, then it has also a closed form that does not involve these functions.

There are expressions in radicals for all solutions of cubic equations (degree 3) and quartic equations (degree 4). The size of these expressions increases significantly with the degree, limiting their usefulness.