True or False (i) Let f : R² —>R be a function. If both df/dx and df/dy exist at ā∈R², then any directional derivative of f also exists at ā (ii) Let D be a closed and bounded region in R² and let f be a C¹ function over D. Suppose that f attains its minimum over D at a point ā ∈ D, then ▽f (ā) = 0. (iii) Let A be a subset of R². If A is not open, then A is closed. (iv) Let F be a C² vector field on R³. Then div(curl F) must be the zero vector field. (v) Let f be a C² function on Rⁿ and let ā be a critical point of f. If Hess f(ā) has both positive and negative eigenvalues, then ā cannot be a local minimum or maximum of f.

Calculus: Early Transcendentals
8th Edition
ISBN:9781285741550
Author:James Stewart
Publisher:James Stewart
Chapter1: Functions And Models
Section: Chapter Questions
Problem 1RCC: (a) What is a function? What are its domain and range? (b) What is the graph of a function? (c) How...
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True or False (i) Let f : R² —>R be a function. If both df/dx and df/dy exist at ā∈R², then any directional derivative of f also exists at ā (ii) Let D be a closed and bounded region in R² and let f be a C¹ function over D. Suppose that f attains its minimum over D at a point ā ∈ D, then ▽f (ā) = 0. (iii) Let A be a subset of R². If A is not open, then A is closed. (iv) Let F be a C² vector field on R³. Then div(curl F) must be the zero vector field. (v) Let f be a C² function on Rⁿ and let ā be a critical point of f. If Hess f(ā) has both positive and negative eigenvalues, then ā cannot be a local minimum or maximum of f.
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