Let f : R" → R be a continuously differentiable function, while æ € R" is its non-stationary point (i.e., Vƒ(x) # 0). Moreover, let d* be the vector defined by Vf(x) ||Vf(x)||' d* while d is any unit vector in R" different from d* (i.e., d e R", ||d|| = 1, d # d*). Show that there exist real numbers d e (0, 0), ɛ e (0, ∞) such that f(x + td") – f(x + td) < -ôt (1) for all t e [0, ɛ). Remark: This question is a slightly extended version of Lemma 6.1, Chapter I.2. Hints: (i) Select & as = -(Vf(x))" (d* – d) | and use the Cauchy-Schwartz inequality to show 8 > 0. (ii) For t z 0, approximate f(x+td*) and f(x+td) using the first-order Taylor polymomial of f(x) at x.

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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2.
Let f : R" → R be a continuously differentiable function, while a E R" is its
non-stationary point (i.e., Vf(x) # 0). Moreover, let d* be the vector defined by
Vf(x)
||Vf(x)||'
d* = -
while d is any unit vector in R" different from d* (i.e., d e R", ||d|| = 1, d + d*). Show
that there exist real numbers d E (0, 00), ɛ E (0, 0) such that
%3D
f(x + td*) – f(x + td) < -ôt
(1)
for all t e [0, ɛ).
Remark: This question is a slightly extended version of Lemma 6.1, Chapter I.2.
Hints: (i) Select & as
8 = -; (Vf(x))" (d* – d)
and use the Cauchy-Schwartz inequality to show 8 > 0.
(ii) For t z 0, approximate f(x+td*) and f(x+td) using the first-order Taylor polynomial
of f(x) at x.
Transcribed Image Text:2. Let f : R" → R be a continuously differentiable function, while a E R" is its non-stationary point (i.e., Vf(x) # 0). Moreover, let d* be the vector defined by Vf(x) ||Vf(x)||' d* = - while d is any unit vector in R" different from d* (i.e., d e R", ||d|| = 1, d + d*). Show that there exist real numbers d E (0, 00), ɛ E (0, 0) such that %3D f(x + td*) – f(x + td) < -ôt (1) for all t e [0, ɛ). Remark: This question is a slightly extended version of Lemma 6.1, Chapter I.2. Hints: (i) Select & as 8 = -; (Vf(x))" (d* – d) and use the Cauchy-Schwartz inequality to show 8 > 0. (ii) For t z 0, approximate f(x+td*) and f(x+td) using the first-order Taylor polynomial of f(x) at x.
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