Dini derivative
In mathematics and, specifically, real analysis, the Dini derivatives (or Dini derivates) are a class of generalizations of the derivative. They were introduced by Ulisse Dini.
The upper Dini derivative, which is also called an upper right-hand derivative,[1] of a continuous function
is denoted by and defined by
where is the supremum limit and the limit is a one-sided limit. The lower Dini derivative, , is defined by
where is the infimum limit.
If is defined on a vector space, then the upper Dini derivative at in the direction is defined by
If is locally Lipschitz, then is finite. If is differentiable at , then the Dini derivative at is the usual derivative at .
Remarks
- Sometimes the notation is used instead of and is used instead of [1]
- Also,
and
- So when using the notation of the Dini derivatives, the plus or minus sign indicates the left- or right-hand limit, and the placement of the sign indicates the infimum or supremum limit.
- On the extended reals, each of the Dini derivatives always exist; however, they may take on the values or at times (i.e., the Dini derivatives always exist in the extended sense).
See also
References
- In-line references
- 1 2 Khalil, H.K. (2002). Nonlinear Systems (3rd ed.). Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-067389-7.
- General references
- Lukashenko, T.P. (2001), "Dini derivative", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4.
- Royden, H.L. (1968). Real analysis (2nd ed.). MacMillan. ISBN 978-0-02-404150-0.
- Brian S. Thomson; Judith B. Bruckner; Andrew M. Bruckner (2008). Elementary Real Analysis. ClassicalRealAnalysis.com [first edition published by Prentice Hall in 2001]. pp. 301–302. ISBN 978-1-4348-4161-2.
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