Another really basic question... this time regarding integration.


by Mathguy15
Tags: basic, integration, time
Mathguy15
Mathguy15 is offline
#1
Jun13-12, 09:20 PM
P: 63
Given a function f define a new function Sf(x) by summing up all values of f(hj)
where 0 ≤ jh < x. That is, if k is such that kh is the largest below x, then
Sf(x) = h[ f(0) + f(h) + f(2h) + .... + f(kh) ]
We call Sf also the ”integral” or ”antiderivative” of f.

The teacher who wrote the lecture notes I'm reading through gives an example of integration. He evaluates Sf(x) for f(x)=1. I don't understand the first sentence:

We have Sf(x) = 0 for x ≤ h.

Why? Sorry for being such a n00b, but I don't understand. Please help me.

Thanks,
Mathguy

By the way, he verifies that the js in the definition are integers.
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pwsnafu
pwsnafu is offline
#2
Jun13-12, 10:52 PM
Sci Advisor
P: 773
Quote Quote by Mathguy15 View Post
We call Sf also the ”integral” or ”antiderivative” of f.
Err, the integral is what you get if you take the limit h -> 0.
Is that word-for-word what is written there?

We have Sf(x) = 0 for x ≤ h.
That doesn't seem right. jh is allowed to equal 0, so the largest integer k such that
0 ≤ kh < x ≤ h is when k=0. So Sf(x) = h f(0) = h.

Edit: Maybe he means x < 0?
Mathguy15
Mathguy15 is offline
#3
Jun14-12, 09:56 AM
P: 63
Well, Yes, that is word-for-word, but I think he's doing a "preliminary" definition before the real definition. And I was thinking the same thing, because Sf(x) isn't defined for x<0.


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