In mathematics, a zero of a real-, complex-, or generally vector-valued function, is a member of the domain of such that vanishes at ; that is, the function attains the value of 0 at, or equivalently, is the solution to the equation. A "zero" of a function is thus an input value that produces an output of. A root of a polynomial is a zero of the corresponding polynomial function. The fundamental theorem of algebra shows that any non-zero polynomial has a number of roots at most equal to its degree, and that the number of roots and the degree are equal when one considers the complex roots counted with their multiplicities. For example, the polynomial of degree two, defined by has the two roots and, since If the function maps real numbers to real numbers, then its zeros are the -coordinates of the points where its graph meets the x-axis. An alternative name for such a point in this context is an -intercept.
Solution of an equation
Every equation in the unknown may be rewritten as by regrouping all the terms in the left-hand side. It follows that the solutions of such an equation are exactly the zeros of the function. In other words, a "zero of a function" is precisely a "solution of the equation obtained by equating the function to 0", and the study of zeros of functions is exactly the same as the study of solutions of equations.
Polynomial roots
Every real polynomial of odd degree has an odd number of real roots ; likewise, a real polynomial of even degree must have an even number of real roots. Consequently, real odd polynomials must have at least one real root, whereas even polynomials may have none. This principle can be proven by reference to the intermediate value theorem: since polynomial functions are continuous, the function value must cross zero, in the process of changing from negative to positive or vice versa.
The fundamental theorem of algebra states that every polynomial of degree has complex roots, counted with their multiplicities. The non-real roots of polynomials with real coefficients come in conjugate pairs. Vieta's formulas relate the coefficients of a polynomial to sums and products of its roots.
Computing roots
Computing roots of functions, for example polynomial functions, frequently requires the use of specialised or approximation techniques. However, some polynomial functions, including all those of degree no greater than 4, can have all their roots expressed algebraically in terms of their coefficients.
Zero set
In various areas of mathematics, the zero set of a function is the set of all its zeros. More precisely, if is a real-valued function, its zero set is, the inverse image of in. The term zero set is generally used when there are infinitely many zeros, and they have some non-trivial topological properties. For example, a level set of a function is the zero set of. The cozero set of is the complement of the zero set of .