# Difference between revisions of "Banach indicatrix"

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N(y,f) = +\infty, | N(y,f) = +\infty, | ||

$$ | $$ | ||

− | |||

and if it has no roots, then | and if it has no roots, then | ||

− | + | $$ | |

+ | N(y,f) = 0. | ||

+ | $$ | ||

The function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b0151509.png" /> was defined by S. Banach [[#References|[1]]] (see also [[#References|[2]]]). He proved that the indicatrix <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b01515010.png" /> of any continuous function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b01515011.png" /> in the interval <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b01515012.png" /> is a function of Baire class not higher than 2, and | The function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b0151509.png" /> was defined by S. Banach [[#References|[1]]] (see also [[#References|[2]]]). He proved that the indicatrix <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b01515010.png" /> of any continuous function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b01515011.png" /> in the interval <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b015/b015150/b01515012.png" /> is a function of Baire class not higher than 2, and |

## Revision as of 03:07, 15 May 2015

*multiplicity function, of a continuous function $y=f(x)$, $a\leq x\leq b$*

An integer-valued function $N(y,f)$, $-\infty < y < \infty$, equal to the number of roots of the equation $f(x)=y$. If, for a given value of $y$, this equation has an infinite number of roots, then $$ N(y,f) = +\infty, $$

and if it has no roots, then

$$ N(y,f) = 0. $$

The function was defined by S. Banach [1] (see also [2]). He proved that the indicatrix of any continuous function in the interval is a function of Baire class not higher than 2, and

(*) |

where is the variation of on . Thus, equation (*) can be considered as the definition of the variation of a continuous function . The Banach indicatrix is also defined (preserving equation (*)) for functions with discontinuities of the first kind [3]. The concept of a Banach indicatrix was employed to define the variation of functions in several variables [4], [5].

#### References

[1] | S. Banach, "Sur les lignes rectifiables et les surfaces dont l'aire est finie" Fund. Math. , 7 (1925) pp. 225–236 |

[2] | I.P. Natanson, "Theorie der Funktionen einer reellen Veränderlichen" , H. Deutsch , Frankfurt a.M. (1961) (Translated from Russian) |

[3] | S.M. Lozinskii, "On the Banach indicatrix" Vestnik Leningrad. Univ. Math. Mekh. Astr. , 7 : 2 pp. 70–87 (In Russian) |

[4] | A.S. Kronrod, "On functions of two variables" Uspekhi Mat. Nauk , 5 : 1 (1950) pp. 24–134 (In Russian) |

[5] | A.G. Vitushkin, "On higher-dimensional variations" , Moscow (1955) (In Russian) |

#### Comments

More generally, for any mapping define analogously. Then, let be a separable metric space and let be -measurable for all Borel subsets of . Let for and let be the measure on defined by the Carathéodory construction from . Then

for every Borel set . Cf. [a1], p. 176 ff. For significant extension of (*), cf. [a2].

#### References

[a1] | H. Federer, "Geometric measure theory" , Springer (1969) |

[a2] | H. Federer, "An analytic characterization of distributions whose partial derivatives are representable by measures" Bull. Amer. Math. Soc. , 60 (1954) pp. 339 |

**How to Cite This Entry:**

Banach indicatrix.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Banach_indicatrix&oldid=36408