- Algebraic element
In
mathematics , the roots ofpolynomial s are inabstract algebra called algebraic elements. They can be created in a larger structure ('adjoined'), not simply found to exist in a given one.More precisely, if "L" is a
field extension of "K" then an element "a" of "L" is called an algebraic element over "K", or just algebraic over "K", if there exists some non-zeropolynomial "g"("x") withcoefficient s in "K" such that "g"("a")=0. Elements of "L" which are not algebraic over "K" are called transcendental over "K".These notions generalize the
algebraic number s and thetranscendental number s (where the field extension is C/Q, C being the field ofcomplex number s and Q being the field ofrational number s).Examples
* The
square root of 2 is algebraic over Q, since it is the root of the polynomial "g"("x") = "x"2 - 2 whose coefficients are rational.
*Pi is transcendental over Q but algebraic over the field ofreal number s R: it is the root of "g"("x") = "x" - π, whose coefficients (1 and -π) are both real, but not of any polynomial with only rational coefficients. (The definition of the termtranscendental number uses C/Q, not C/R.)Properties
The following conditions are equivalent for an element "a" of "L":
* "a" is algebraic over "K"
* the field extension "K"("a")/"K" has finite degree, i.e. the dimension of "K"("a") as a "K"-vector space is finite. (Here "K"("a") denotes the smallest subfield of "L" containing "K" and "a")
* "K" ["a"] = "K"("a"), where "K" ["a"] is the set of all elements of "L" that can be written in the form "g"("a") with a polynomial "g" whose coefficients lie in "K".This characterization can be used to show that the sum, difference, product and quotient of algebraic elements over "K" are again algebraic over "K". The set of all elements of "L" which are algebraic over "K" is a field that sits in between "L" and "K".
If "a" is algebraic over "K", then there are many non-zero polynomials "g"("x") with coefficients in "K" such that "g"("a") = 0. However there is a single one with smallest degree and with leading coefficient 1. This is the
minimal polynomial of "a" and it encodes many important properties of "a".Fields that do not allow any algebraic elements over them (except their own elements) are called algebraically closed. The field of complex numbers is an example.
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