# Viète theorem

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on roots

A theorem which establishes relations between the roots and the coefficients of a polynomial. Let $f( x)$ be a polynomial of degree $n$ with coefficients from some field and with leading coefficient 1. The polynomial $f( x)$ splits over a field containing all the roots of $f$( e.g. over the splitting field of $f( x)$, cf. Splitting field of a polynomial) into linear factors:

$$f ( x) = x ^ {n} + a _ {n-} 1 x ^ {n-} 1 + \dots + a _ {1} x + a _ {0\ } =$$

$$= \ ( x - \alpha _ {1} ) \dots ( x - \alpha _ {n} ),$$

where $\alpha _ {i}$ are the roots of $f( x)$, $i = 1 \dots n$. Viète's theorem asserts that the following relations (Viète's formulas) hold:

$$a _ {0} = (- 1) ^ {n} \alpha _ {1} \dots \alpha _ {n} ,$$

$$a _ {1} = (- 1) ^ {n-} 1 ( \alpha _ {1} \alpha _ {2} \dots \alpha _ {n-} 1 + \alpha _ {1} \dots \alpha _ {n-} 2 \alpha _ {n} + \dots$$

$$\dots {} + \alpha _ {2} \alpha _ {3} \dots \alpha _ {n} ),$$

$$\dots \dots \dots \dots$$

$$a _ {n-} 2 = \alpha _ {1} \alpha _ {2} + \alpha _ {1} \alpha _ {3} + \dots + \alpha _ {n-} 1 \alpha _ {n} ,$$

$$a _ {n-} 1 = - ( \alpha _ {1} + \dots + \alpha _ {n} ).$$

F. Viète [1] proved this relation for all $n$, but for positive roots only; the general form of Viète's theorem was established by A. Girard [2].

#### References

 [1] F. Viète, "Opera mathematica" F. van Schouten (ed.) , Leiden (1646) [2] A. Girard, "Invention nouvelle en l'algèbre" , Bierens de Haan , Leiden (1884) (Reprint)

#### Comments

A polynomial with leading coefficient $1$ is called monic. Up to sign, the expressions for $\alpha _ {i}$ in Viète's theorem are nowadays known as the (elementary) symmetric polynomials (of $n$ variables; cf. Symmetric polynomial).

Viète's name is sometimes spelled Vièta: Vièta theorem.

#### References

 [a1] B.L. van der Waerden, "Algebra" , 1 , Springer (1967) (Translated from German)
How to Cite This Entry:
Viète theorem. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Vi%C3%A8te_theorem&oldid=49154
This article was adapted from an original article by V.N. Remeslennikov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article