# Homework Help: Differential Equation - Proof - Feedback.

1. Oct 29, 2008

### PFStudent

Hey,

1. The problem statement, all variables and given/known data.
I wrote my own proof for the below, I was wondering if you guys could take a look at it and give me some feedback please. Particularly, I would like to know if this proof is rigorous enough.

Lemma 2.4 - Suppose f has a second derivative everywhere, and that,
$${{f} + {f^{\prime\prime}}} = {0}$$
$${f(0)} = {0}$$
$${f^{\prime\prime}(0)}} = {0}$$

Then,
$${f(x)} = {0}$$

2. Relevant equations.
Knowledge of Calculus.

3. The attempt at a solution.
Let,
$${{f(x)}, {{{f}^{\prime}}{(x)}}, {{{f}^{\prime\prime}}{(x)}},...,} = {{f}, {{f}^{\prime}}, {{f}^{\prime\prime}},...,}$$

Prove that,
$${f} = {0}$$

Proof,
$${{f}+{f^{\prime\prime}}} = {0}$$

$${{\left({{f}^{\prime}}\right)}{\left({{f}+{{f}^{\prime\prime}}}\right)}} = {\left({0}\right)}{\left({{f}^{\prime}}\right)}$$

$${{{f}^{\prime}}{\left({{f}+{{f}^{\prime\prime}}}\right)}} = {0}$$

$${{\int_{}^{}}{\left[{{{f}^{\prime}}{\left({{f}{+{{f}^{\prime\prime}}}}\right)}}\right]}{dx}} = {{\int_{}^{}}{\left[{0}\right]}{dx}} {\textcolor{white}{.}} , {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} Let {\textcolor{white}{.}} {0} = {a}$$

$${{{\int_{}^{}}}{f}{{f}^{\prime}}{dx}+{{\int_{}^{}}{{f}^{\prime}}{{f}^{\prime\prime}}{dx}}} = {{{\int_{}^{}}}{\left({a}\right)}{dx}}$$

Let,
$${\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {u} = {f} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {v} = {{f}^{\prime}}$$

$${du} = {{{f}^{\prime}}{dx}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {dv} = {{{f}^{\prime\prime}}{dx}}$$

$${{{\int_{}^{}}{\left({u}\right)}{\left({du}\right)}}+{{\int_{}^{}}{\left({v}\right)}{\left({dv}\right)}}} = {{\int_{}^{}}{a}{dx}}$$

$${{\left({{\frac{{u}^{2}}{2}}+{{C}_{1}}}\right)}+{\left({{\frac{{v}^{2}}{2}}+{{C}_{2}}}\right)}} = {\left({{ax}+{C}}\right)}$$

Let,
$${{{C}_{1}}+{{C}_{2}}} = {C}$$

$${{{\frac{1}{2}}{\left({{{\left({f}\right)}^{2}}+{{\left({{f}^{\prime}}\right)}^{2}}}\right)}}+{\left({C}\right)}} = {{{\left({0}\right)}{x}}+{C}}$$

$${{\frac{1}{2}}{{\left({{{f}^{2}}+{{{f}^{\prime}}^{2}}}\right)}}} = {0}$$

$${{{\left({{{f}^{2}}+{{{f}^{\prime}}^{2}}}\right)}}} = {0}$$

$${{{\left({f(x)}\right)}^{2}}{+}{{\left({{{f}^{\prime}}{\left({x}\right)}}\right)}^{2}}} = {0}$$
, for all ${x}$.

This implies that $${{f(x)} = {0}}$$ for all ${x}$.

$${\therefore}$$

$${f} = {0}$$

Thanks,

-PFStudent
EDIT: Thanks for the edit HallsofIvy.

Last edited: Oct 30, 2008
2. Oct 29, 2008

### HallsofIvy

You mean $$f(x)= 0$$ for all x.

No. C1, C2, and C are arbitrary constants. You can't just assume they cancel. You can, of course, use the given facts that f(0)=0 and f'(0)= 0.

3. Nov 2, 2008

### PFStudent

Hey,
Thanks, ok I understand I need to use the given facts,
$${f(0)} = {0}$$
$${f^{\prime\prime}(0)}} = {0}$$

However, I'm just not sure how to use those facts. In addition, addressing the point about the constants I reworked the problem to where I am still stuck,

$${{{\int_{}^{}}}{f}{{f}^{\prime}}{dx}+{{\int_{}^{}}{{f}^{\prime}}{{f}^{\prime\prime}}{dx}}} = {{{\int_{}^{}}}{\left({a}\right)}{dx}}$$

Let,
$${\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {u} = {f} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {v} = {{f}^{\prime}}$$

$${du} = {{{f}^{\prime}}{dx}} {\textcolor{white}{.}} {\textcolor{white}{.}} {\textcolor{white}{.}} {dv} = {{{f}^{\prime\prime}}{dx}}$$

$${{{\int_{}^{}}{\left({u}\right)}{\left({du}\right)}}+{{\int_{}^{}}{\left({v}\right)}{\left({dv}\right)}}} = {{\int_{}^{}}{a}{dx}}$$

$${{\left({{\frac{{u}^{2}}{2}}+{{C}_{1}}}\right)}+{\left({{\frac{{v}^{2}}{2}}+{{C}_{2}}}\right)}} = {\left({{ax}+{{C}_{3}}}\right)}$$

$${{{\frac{1}{2}}{\left({{{\left({f}\right)}^{2}}+{{\left({{f}^{\prime}}\right)}^{2}}}\right)}}+{{C}_{1}}+{{C}_{2}}} = {{{\left({0}\right)}{x}}+{{C}_{3}}}$$

$${{\frac{1}{2}}{{\left({{{f}^{2}}+{{{f}^{\prime}}^{2}}}\right)}}+{{C}_{1}}+{{C}_{2}}} = {{C}_{3}}$$

I'm not sure from here how to use the given facts to finish the proof, so any help is appreciated.

Thanks,

-PFStudent

Last edited: Nov 2, 2008
4. Nov 2, 2008

### HallsofIvy

Which is the same as saying
$${{\frac{1}{2}}{{\left({{{f}^{2}}+{{{f}^{\prime}}^{2}}}\right)}} = C$$
Since, while you cannot assume the constants cancel, you can combine them into a single constant. Now use the fact that f(0)= f"(0)= 0 to determine C.

5. Nov 2, 2008

### PFStudent

Hey,
Thanks, ok so combining the constants I end up with the following,
$${{\frac{1}{2}}{{\left({{{f}^{2}}+{{{f}^{\prime}}^{2}}}\right)}}+{{C}_{1}}+{{C}_{2}}} = {{C}_{3}}$$

Let,
$${C} = {{{C}_{3}}-{\left({{{C}_{1}}+{{C}_{2}}}\right)}}$$

$${{\frac{1}{2}}{{\left({{{f}^{2}}+{{{f}^{\prime}}^{2}}}\right)}}} = {\left({C}\right)}$$

$${{\frac{1}{2}}{{\left({f(x)}\right)}^{2}}+{{\left({{{f}^{\prime}}{\left({x}\right)}}\right)}^{2}}} = {C}$$

Err, it seems I made a typo in the beginning rather than,
$${{{f}^{\prime\prime}}{(0)}} = {0}$$

It should be,
$${{{f}^{\prime}}{(0)}} = {0}$$

That being said, I do not think that changes the problem significantly. So, going back to using the facts given,
$${f(0)} = {0}$$
$${{{f}^{\prime}}{(0)}} = {0}$$

While I am familiar with solving differential equations using initial conditions and I think that is what is being hinted at in this step. I'm not quite sure how I would use the two equations above to solve for the constant $${C}$$. Maybe I would integrate the $${{f}^{\prime}}$$ function but that doesn't make since, because I only know the value of the function when $${{x} = {0}}$$. A little more elaboration would be greatly appreciated, thanks for the help.

Thanks,

-PFStudent