algebraic function

A function  of one variable is said to be algebraic if it satisfies a polynomial equation whose coefficients  are polynomials   in the same variable. Namely, the function $f(x)$ is algebraic if $y=f(x)$ is a solution of an equation of the form

 $p_{n}(x)y^{n}+\cdots+p_{1}(x)y+p_{0}(x)=0,$

where the $p_{0}(x),p_{1}(x),\ldots,p_{n}(x)$ are polynomials in $x$. A function that satisfies no such equation is said to be transcendental.

Examples

Any rational function  $f(x)=P(x)/Q(x)$ is algebraic, since $y=f(x)$ is a solution to $Q(x)y-P(x)=0$.

The function $f(x)=\sqrt{x}$ is algebraic, since $y=f(x)$ is a solution to $y^{2}-x=0$. The same is true for any power function   $x^{n/m}$, with $n$ and $m$ integers, it satisfies the equation $y^{m}-x^{n}=0$.

Remark. There is also a version of an algebraic function defined on algebraic systems. Given an algebraic system $A$, an $n$-ary algebraic function on $A$ is an $n$-ary operator $f(x_{1},\ldots,x_{n})$ on $A$ such that there is an $(n+m)$-ary polynomial (http://planetmath.org/PolynomialsInAlgebraicSystems) $p(x_{1},\ldots,x_{n},x_{n+1},\ldots,x_{n+m})$ on $A$ for some non-negative integer $m$, and elements $a_{1},\ldots,a_{m}\in A$ such that

 $f(x_{1},\ldots,x_{n})=p(x_{1},\ldots,x_{n},a_{1},\ldots,a_{m}).$

Equivalently, given an algebraic system $A$, if we associate each element $a$ of $A$ a corresponding symbol, also written $a$, we may form an algebraic system $A^{\prime}$ from $A$ by adjoining every symbol $a$ to the type of $A$ considered as a unary operator symbol, and leaving everything else the same. Then an algebraic function on $A$ is just a polynomial on $A^{\prime}$ (and vice versa).

For example, in a ring $R$, a function $f$ on $R$ given by $f(x)=a_{n}x^{n}+\cdots+a_{1}x+a_{0}$ where $a_{i}\in R$ is a unary algebraic function on $R$, as $f(x)=p(x,a_{0},\ldots,a_{n})$, where $p$ is an $(n+2)$-ary polynomial on $R$ given by $p(x,x_{0},\ldots,x_{n})=x_{n}x^{n}+\cdots+x_{1}x+x_{0}$.

References

• 1 G. Grätzer: , 2nd Edition, Springer, New York (1978).
• 2 S. Burris, H.P. Sankappanavar: A Course in Universal Algebra, Springer, New York (1981).
 Title algebraic function Canonical name AlgebraicFunction Date of creation 2013-03-22 15:19:24 Last modified on 2013-03-22 15:19:24 Owner CWoo (3771) Last modified by CWoo (3771) Numerical id 15 Author CWoo (3771) Entry type Definition Classification msc 08A40 Classification msc 26A09 Related topic ElementaryFunction Related topic PropertiesOfEntireFunctions Related topic PolynomialsInAlgebraicSystems Defines transcendental function Defines transcendental Defines algebraic curve