# Find the supremum, infinum, maximum and minimum

1. Aug 2, 2011

### joshuamcevoy

1. The problem statement, all variables and given/known data

find the supremum, infinum, maximum and minimum
2. Relevant equations
(n-2sqrt(n)) n is element of natural numbers

3. The attempt at a solution
no idea on how to do this help please

2. Aug 2, 2011

Re: sup,inf,max,min

Drawing the graph is always a good idea. You know the graph of y=x and i assume also the graph of 2*x^1/2. After that you can easily sketch x-2*x^2 and then you'll probably have a better idea were to look

3. Aug 2, 2011

### Disconnected

Re: sup,inf,max,min

Do you know the definition of sup, inf, max and min, or is it just this question in particular that is causing problems?

4. Aug 2, 2011

### joshuamcevoy

Re: sup,inf,max,min

its this question in particular that's giving me the headache

5. Aug 2, 2011

### Disconnected

Re: sup,inf,max,min

Well, what is the smallest natural number? What is the value of the function at that point? Does the function increase or decrease from that point? Does it converse or diverge?

6. Aug 2, 2011

### spamiam

Re: sup,inf,max,min

Why don't you try finding the critical points for the related function $f(x) = x - 2 \sqrt{x}$ for real numbers and then restricting your answer to the naturals? This would also provide you with an easy way to use Disconnected's idea of examining when the function is increasing and decreasing.