MAT237 Multivariable Calculus with Proofs Problem Set 2
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MAT237 Multivariable Calculus with Proofs
Problem Set 2
Problems
1. (1a) Give an example of a non-convergent sequence {xk)in Rn such that the set {x1 , x2 , ...)is not closed.
(1b) Let {xk)be a sequence in Rn with no convergent subsequence. Show that the set {x1 , x2 , ...)is closed.
2. Let A, B - Rm and let f : A u B o Rn . Assume the restricted functions: f |A : A o Rn and f |B : B o Rn are continuous.
(2a) Give an example of a function f and sets A and B such that f is not continuous.
(2b) Prove that if A and B are closed, then f is continuous.
3. Let A, B - Rn be non-empty path-connected sets.
(3a) Prove that if A n B a then A u B is path connected.
(3b) Give an example of non-empty sets A and B such that A n B = a and A u B is path connected.
(3c) Prove that if A and B are non-empty open subsets of Rn and An B = a then Au B is not path connected.
4. (4a) Let A S Rn and f : A o Rm . Suppose a is a limit point of A. Prove that if there exist parametric curves 51 : [0, 1] o Rn and 52 : [0, 1] o Rn and b1 , b2 e (0, 1) such that:
• 51 (t) a if t b1 and 52 (t) a if t b2 ,
• tlob1(im) 51 (t) = tlob2(im) 52 (t) = a,
• and tlob1(im) f (51 (t)) tlob2(im) f (52 (t)),
then x(l)oa(im)f (x) does not exist.
(4b) Let f (x , y) = a . Use part (a) to prove that (x ,y(li)0,0)f (x , y) does not exist.
5. Limits can measure the rate at which functions tend to zero (or infinity) and polynomials are your favourite functions of all time. Let y1 , . . . , yn e N and let A e N+ . You will compare the behaviours of the monomial x 1(y)1 ﹒ ﹒ ﹒ x n(y)n and the norm ||x||A as l(x1 , . . . , xn )l o o.
(5a) Prove that if n c 2 and y1 + ﹒ ﹒ ﹒ + yn c A, then lxl(li)oo(m) does not exist. Do not use the result
(5b) Briefly explain why the assumption n c 2 is necessary is part (a).
(5c) Use the e _ δ definition of the limit to prove that if y1 + ﹒ ﹒ ﹒ + yn < A, then lxl(li)oo(m) lxlA = 0.
6. The Intermediate Value Theorem tells us that if f : R o R is continuous on [a, b] and f (a) s c s f (b), then c is in the range of f . Is the same true for functions f : A o R where A S Rn ? We need an additional assumption in this case: that A is path-connected. We will prove the following result:
Let A S Rn be a path-connected set and a, b e A. Suppose f : A o R and c e R such that f (a) s c s f (b).
(6a) Consider the following proof of the claim:
1. Since f is continuous on A and A is path-connected, f (A) is path-connected subset of R. 2. Therefore, f (A) is an interval. 3. Since f (a), f (b) e f (A) and f (a) < f (b), we also have [f (a), f (b)] S f (A). 4. Since c e [f (a), f (b)], there exists x e A such that f (x) = c. |
This proof has a line which is completely unjustified. Identify the line which is missing justification.
口 Line 1 口 Line 2 口 Line 3 口 Line 4
(6b) To justify this line, we must prove a statement about subsets of R. Fill in the blanks to complete this statement:
If B S R is , then B is . (6c) Prove your statement from part (b).
2022-05-28