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Topic: Big-O Notation Estimates
Replies: 2   Last Post: Oct 4, 2012 11:16 AM

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Posts: 3
Registered: 10/3/12
Big-O Notation Estimates
Posted: Oct 3, 2012 8:59 PM
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Im having some trouble understanding Big-O estimates. Im using Discrete Mathematics And Its Applications by Rosen 6th Edition. In section 3.2 one of the example problems says to show that f(x) - x^2 + 2x + 1 is O(x^2).

The solution states the following:

"We observe that we can readily estimate the size of f(x) when x > 1 because x < x^2 and 1 < x^2 when x > 1. It follows that

0 <= x^2 + 2x + 1 <= x^2 + 2x^2 + x^2 = 4x^2

whenever x > 1.
Consequently, we can take C = 4, k = 1 as witnesses to show that f(x) is O(x^2). That is, f(x) = x^2 + 2x + 1 = 4x^2 whenever x > 1.

I have read this so many times and for some reason the concept is not sticking. Im hoping someone can explain how this problem was worked. Here is what I dont understand:

Why is "0<=" being used here?
How did we go from x^2 + 2x + 1 to x^2 + 2x^2 + x^2?

I thought maybe we are using the value of g of x (x^2) to replace the x values in the f(x) function but should this not be x^4 + 2x^2 + 1?

Any help would be appreciated.

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