Weak Derivatives
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Lecture26
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Yuliya Gorb Weak Derivatives Lecture 26 November 26, 2013 Lecture 26 Weak Derivatives Yuliya Gorb Weak Derivatives Suppose Ω ⊂ R n is an open set. Definition A function f ∈ L 1 loc
(Ω) is weakly differentiable w.r.t. x i if there exists a function g i ∈ L 1 loc (Ω) s.t. Z Ω f ∂φ ∂x i dx = − Z Ω g i φ dx,
for all φ ∈ C ∞ c (Ω). The function g i is called the weak ith partial derivative of f , and denoted ∂f ∂x
. Since C
∞ c (Ω) is dense in L 1 loc
(Ω), the weak derivative of a function, if it exists, is unique up to pointwise (p.w.) almost everywhere (a.e.) equivalence. The weak derivative of a continuously differentiable function coincides with the p.w. derivative. However, the existence of a weak derivative is not equivalent to the existence of a p.w. derivative a.e. Lecture 26 Weak Derivatives
Yuliya Gorb Weak Derivatives (cont.) Definition Suppose that α ∈ N n 0
1 loc
(Ω) has weak derivative D α f
∂ α f ∂x α i ∈ L 1 loc (Ω) if Z Ω ∂ α f ∂x α i φ dx = (−1)
|α| Z Ω f ∂ α φ ∂x α i dx , for all φ ∈ C ∞ c (Ω). Lecture 26 Weak Derivatives
Yuliya Gorb Examples
Ex. 1 (n=1): Continuous function and weakly differentiable Consider f (x) = |x| ∈ C (R). Then f is weakly differentiable, with f ′
( −1,
x < 0 1, x ≥ 0
Ex. 2 (n=1): Discontinuous function f : R → R The function f (x) =
( −1, x < 0 1, x
is not
weakly differentiable. Lecture 26 Weak Derivatives
Yuliya Gorb Examples (cont.) Ex. 3 (n=1): Continuous but not weakly differentiable Consider a continuous function that is p.w. differentiable a.e. but does not have a weak derivative: the
Cantor function c (x) ∈ C [0, 1] Ex. 4: n-dim For a ∈ R define f : R n → R by
f (x) =
1 |x|
a is weakly differentiable if a + 1 < n with weak derivative ∂f ∂x
(x) = − a |x| a +1 x i |x|
Lecture 26 Weak Derivatives Yuliya Gorb References Evans pp. 256–258 Lecture 26 Weak Derivatives Document Outline
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