Entire function

Definition

In the mathematical analysis and, in particular, in the theory of functions of complex variable, The entire function is function that is holomorphic in the whole complex plane .

Examples

Entires

Examples of entire functions are the polynomials and the exponentials. All sums, products and compositions of these functions also are entire functions.

All the derivatives and some of integrals of entired funcitons, for example erf, Si, $J_{0}$, also are entired functions.

Non-entires

In general, neither series nor limit of a sequence of entire funcitons needs to be an entire function.

Inverse of an entire function has no need to be entire function. Usually, invetse of a non-trivial function is not entire. (The inverse of the linear function is entire). In particular, inverse of trigonometric functions are not entire.

More non-entire functions: rational function $~f(z)={\frac {a+bx}{c+x}}~$ at any complex $~a~$, $~b~$, $~c~$ , square root, logarithm, function Gamma, tetration.

In particular, non-analytic functions also should be qualified as non-entire: $\Re$, $\Im$, complex conjugation, modulus, argument, Dirichlet function.

Properties

Power series

The radius of convergence of a power series is always distance until the nearest singulatity. Therefore, it is infinite for entire functions. Every entire function can be represented as a power series or Tailor expansion which converges everywhere.

Infinitness

Liouville's theorem establishes an important property of entire functions — an entire function which is bounded must be constant .

Range of values

Picard's little theorem states: a non-constant entire function takes on every complex number as value, except possibly one .

For example, the exponential never takes on the value 0.

Cauchi integral

Entire function $~f~$, at any complex $~z~$ and at any contour C evolving point $z$ just once, can be expressed with Cauchi theorem

$f(x)={\frac {1}{2\pi {\rm {i}}}}\oint _{\mathbf {C} }{\frac {f(t)}{t-z}}{\rm {d}}t$

See also

• Cauchi formula
• Tailor series