# Smoothness, modulus of

The modulus of continuity of the derivative of order of a function defined on a Banach space , i.e. the expression

where . If , the modulus of smoothness is the ordinary modulus of continuity (cf. Continuity, modulus of) of . Basic properties of the modulus of smoothness (in the case and the space of continuous functions) are:

does not decrease together with ;

if is an integer, then

for any ,

if , then

if , then

where and are constants independent of .

Certain problems in the theory of approximation of functions can ultimately be solved only in terms of a modulus of smoothness of order . In the theory of approximations of functions an important class is the class of periodic continuous functions with period and with second-order modulus of smoothness satisfying the condition

The modulus of continuity of such functions satisfies the condition

, and the constant cannot be improved [4].

#### References

[1] | S.N. Bernshtein, "Sur l'ordre de la meilleure approximation des fonctions continués par de polynomes de degré donné" Mem. Publ. Classe Sci. Acad. Belgique (2) , 4 (1912) pp. 1–103 |

[2] | A. Marchaud, "Sur les dérivées et sur les différences des fonctions de variables réelles" J. Math. Pures Appl. , 6 (1927) pp. 337–425 |

[3] | A. Zygmund, "Smooth functions" Duke Math. J. , 12 (1945) pp. 47–76 |

[4] | A.V. Efimov, "Estimate of the modules of continuity of a function in the class " Izv. Akad. Nauk SSSR Ser Mat. , 21 (1957) pp. 283–288 (In Russian) |

#### Comments

The modulus of smoothness is also written in terms of symmetric differences, as , where

and

This gives a recurrent procedure for computing (approximations of) it.

To overcome certain shortcomings of this (classical) modulus of smoothness (especially its ability to characterize the order of the best polynomial approximation to a function ), new moduli of smoothness have been introduced. They use so-called step-weight functions , and are defined by

The function is chosen for the problem at hand. Note that here the increment varies with . A basic result is that if and only if . (Here , , , , and approximation is in .) For more on such moduli, their use in approximation problems and in the interpolation of spaces, see [a1].

#### References

[a1] | Z. Ditzian, V. Totik, "Moduli of smoothness" , Springer (1987) |

[a2] | G.G. Lorentz, "Approximation of functions" , Holt, Rinehart & Winston (1966) |

**How to Cite This Entry:**

Smoothness, modulus of.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Smoothness,_modulus_of&oldid=12777