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In ], a '''zero''' of a ] ''f'' is a ] ''a'' such that ''f''(''a'') = 0. In ], a '''zero''' of a ] ''f'' is a ] ''a'' such that ''f''(''a'') = 0. (See also ].)


==Multiplicity of a zero== ==Multiplicity of a zero==

Revision as of 22:11, 7 January 2005

In complex analysis, a zero of a holomorphic function f is a complex number a such that f(a) = 0. (See also root (mathematics).)

Multiplicity of a zero

A complex number a is a simple zero of f, or a zero of multiplicity 1 of f, if a is not a zero of the holomorophic function g such that

f ( z ) = ( z a ) g ( z ) . {\displaystyle f(z)=(z-a)g(z).}

Generally, the multiplicity of the zero of f at a is the positive integer n for which there is a holomorphic function g such that

f ( z ) = ( z a ) n g ( z )   and   g ( a ) 0. {\displaystyle f(z)=(z-a)^{n}g(z)\ {\mbox{and}}\ g(a)\neq 0.}

Existence of zeroes

The so-called fundamental theorem of algebra (something of a misnomer) says that every nonconstant polynomial function with complex coefficients has at least one zero in the complex plane. This is in contrast to the situation with real zeroes: some polynomial functions with real coefficients have no real zeroes (but since real numbers are complex numbers, they still have complex zeroes). An example is f(x) = x

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