# inequality of supremums

• Apr 26th 2009, 07:54 AM
deubelte
inequality of supremums
Hi all,

If $\displaystyle f,g$ and $\displaystyle h$ are 3 continous functions on a set A,
that verify:

$\displaystyle \sup({f(t),t\in A})<\sup({h(t),t\in A})+\sup({g(t),t\in A})$

(beware the $\displaystyle <$ and not $\displaystyle \leq$)

can I say that there exists $\displaystyle x\in A$ such that:
$\displaystyle f(x)<g(x)+h(x)$

thank you

Deubelte
• Apr 26th 2009, 08:28 AM
Plato
Let $\displaystyle A=\mathbb{R}$ now define $\displaystyle f(x)=4,~g(x)=1,~\&~h(x)=2$.
Is that statement still true.
You must have left something out of the statement of the question.
• Apr 26th 2009, 09:24 AM
Opalg
Quote:

Originally Posted by deubelte
If $\displaystyle f,g$ and $\displaystyle h$ are 3 continuous functions on a set A,
that verify:

$\displaystyle \sup({f(t),t\in A})<\sup({h(t),t\in A})+\sup({g(t),t\in A})$

(beware the $\displaystyle <$ and not $\displaystyle \leq$)

can I say that there exists $\displaystyle x\in A$ such that:
$\displaystyle f(x)<g(x)+h(x)$

No. Take $\displaystyle A = \mathbb{R}$ and define $\displaystyle g(x) = \frac1{(x-10)^2+1}$, $\displaystyle h(x) = \frac1{(x+10)^2+1}$ and $\displaystyle f(x) = 1.5$ (constant).

Then $\displaystyle \sup\{g(x)\} = 1$ (attained when x=10) and $\displaystyle \sup\{h(x)\} = 1$ (attained when x=–10), so $\displaystyle 1.5 = \sup\{f(x)\} < \sup\{g(x)\}+\sup\{h(x\} = 2$.

But $\displaystyle x\geqslant0\;\Rightarrow\; h(x)\leqslant0.01$, and $\displaystyle x\leqslant0\;\Rightarrow\; g(x)\leqslant0.01$. So $\displaystyle g(x)+h(x)$ is always less than 1.01, which is less than $\displaystyle f(x)$.
• Apr 26th 2009, 11:22 PM
deubelte
I have an other question. supposing we have two continuous functions
$\displaystyle f$ and $\displaystyle g$
such that:
$\displaystyle \sup(g(t),t\in A)<\sup(h(t),t\in A)$

can I say that:

there exists x in A such that:
1) $\displaystyle g(x)\leq h(x)$

2) $\displaystyle forall\epsilon$ there exists x such that:
$\displaystyle g(x)-\epsilon<h(x)$

thx
• Apr 26th 2009, 11:57 PM
Opalg
Quote:

Originally Posted by deubelte
I have an other question. supposing we have two continuous functions
$\displaystyle f$ and $\displaystyle g$
such that:
$\displaystyle \sup(g(t),t\in A)<\sup(h(t),t\in A)$

can I say that:

there exists x in A such that:
1) $\displaystyle g(x)\leq h(x)$

2) $\displaystyle \forall\epsilon$ there exists x such that:
$\displaystyle g(x)-\epsilon<h(x)$

Choose k with $\displaystyle \sup\{g(t),t\in A\}<k<\sup\{h(t),t\in A\}$. Then there exists x in A such that $\displaystyle h(x)>k$. But $\displaystyle g(x)<k$, and therefore $\displaystyle g(x)<h(x)$.
• Apr 27th 2009, 12:32 AM
deubelte
Ok, so I guess that my second statement is also true ??

Merci l'Anglais.