- #1
Mr.Miyagi
- 47
- 0
The problem statement
Let [itex]f:[a,b]→\mathbb{R}[/itex] be differentiable and assume that [itex]f(a)=0[/itex] and [tex]\left|f'(x)\right|\leq A\left|f(x)\right|, x\in [a,b].[/tex]
Show that [itex]f(x)=0,x\in [a,b][/itex].
The attempt at a solution
It was hinted at that the solution was partly as follows. Let [itex]a \leq x_0 \leq b[/itex]. For all [itex]x\in [a,x_0][/itex] [tex]\left|f(x)\right|\leq (x_0-a)\underset{[a,x_0]}{\sup}\left|f'(x)\right|\leq A(x_0-a)\underset{[a,x_0]}{\sup}\left|f(x)\right|.[/tex] Then we should ask what happens for [itex]A(x_0-a)<1[/itex].
The first inequality follows from the mean value theorem and the second from the given equation. When [itex]A(x_0-a)<1[/itex], then [tex]\left|f(x)\right|< \underset{[a,x_0]}{\sup}\left|f(x)\right|.[/tex] I do not see how this would imply the proposed relation.
EDIT: I have already solved the next problem.
The problem statement
Is the series [tex]\sum^{\infty}_{n=1} \frac{x^n(1-x)}{n}[/tex] uniformly convergent on [itex][0,1][/itex]?
The attempt at a solution
I suspect it is. I want to use the Weierstrass M-test, but obviously the sequence [itex]M_n=1/n[/itex] doesn't work as a bound since [tex]\sum^{\infty}_{n=1} \frac{1}{n}[/tex] does not converge. I can't figure how to produce a tighter bound than [itex]M_n=1/n[/itex].
Let [itex]f:[a,b]→\mathbb{R}[/itex] be differentiable and assume that [itex]f(a)=0[/itex] and [tex]\left|f'(x)\right|\leq A\left|f(x)\right|, x\in [a,b].[/tex]
Show that [itex]f(x)=0,x\in [a,b][/itex].
The attempt at a solution
It was hinted at that the solution was partly as follows. Let [itex]a \leq x_0 \leq b[/itex]. For all [itex]x\in [a,x_0][/itex] [tex]\left|f(x)\right|\leq (x_0-a)\underset{[a,x_0]}{\sup}\left|f'(x)\right|\leq A(x_0-a)\underset{[a,x_0]}{\sup}\left|f(x)\right|.[/tex] Then we should ask what happens for [itex]A(x_0-a)<1[/itex].
The first inequality follows from the mean value theorem and the second from the given equation. When [itex]A(x_0-a)<1[/itex], then [tex]\left|f(x)\right|< \underset{[a,x_0]}{\sup}\left|f(x)\right|.[/tex] I do not see how this would imply the proposed relation.
EDIT: I have already solved the next problem.
The problem statement
Is the series [tex]\sum^{\infty}_{n=1} \frac{x^n(1-x)}{n}[/tex] uniformly convergent on [itex][0,1][/itex]?
The attempt at a solution
I suspect it is. I want to use the Weierstrass M-test, but obviously the sequence [itex]M_n=1/n[/itex] doesn't work as a bound since [tex]\sum^{\infty}_{n=1} \frac{1}{n}[/tex] does not converge. I can't figure how to produce a tighter bound than [itex]M_n=1/n[/itex].
Homework Statement
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