# Generalized derivative

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of function type

An extension of the idea of a derivative to some classes of non-differentiable functions. The first definition is due to S.L. Sobolev (see [1], [2]), who arrived at a definition of a generalized derivative from the point of view of his concept of a generalized function.

Let and be locally integrable functions on an open set in the -dimensional space , that is, Lebesgue integrable on any closed bounded set . Then is the generalized derivative of with respect to on , and one writes , if for any infinitely-differentiable function with compact support in (see Function of compact support)

 (1)

A second, equivalent, definition of the generalized derivative is the following. If can be modified on a set of -dimensional measure zero so that the modified function (which will again be denoted by ) is locally absolutely continuous with respect to for almost-all (in the sense of the -dimensional Lebesgue measure) belonging to the projection of onto the plane , then has partial derivative (in the usual sense of the word) almost-everywhere on . If a function almost-everywhere on , then is a generalized derivative of with respect to on . Thus, a generalized derivative is defined almost-everywhere on ; if is continuous and the ordinary derivative is continuous on , then it is also a generalized derivative of with respect to on .

Generalized derivatives of a higher order are defined by induction. They are independent (almost-everywhere) of the order of differentiation.

There is a third equivalent definition of a generalized derivative. Suppose that for each closed bounded set , the functions and , defined on , have the properties:

and suppose that the functions , and their partial derivatives are continuous on . Then is the generalized partial derivative of with respect to on () (see also Sobolev space).

From the point of view of the theory of generalized functions, a generalized derivative can be defined as follows: Suppose one is given a function that is locally summable on , considered as a generalized function, and let be the partial derivative in the sense of the theory of generalized functions. If represents a function that is locally summable on , then is a generalized derivative (in the first (original) sense).

The concept of a generalized derivative had been considered even earlier (see [3] for example, where generalized derivatives with integrable square on are considered). Subsequently, many investigators arrived at this concept independently of their predecessors (on this question see [4]).

#### References

 [1] S.L. Sobolev, "Le problème de Cauchy dans l'espace des fonctionnelles" Dokl. Akad. Nauk SSSR , 3 : 7 (1935) pp. 291–294 [2] S.L. Sobolev, "Méthode nouvelle à résoudre le problème de Cauchy pour les équations linéaires hyperboliques normales" Mat. Sb. , 1 (1936) pp. 39–72 [3] B. Levi, "Sul principio di Dirichlet" Rend. Circ. Mat. Palermo , 22 (1906) pp. 293–359 [4] S.M. Nikol'skii, "Approximation of functions of several variables and imbedding theorems" , Springer (1975) (Translated from Russian)