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

by Mathguy15
Tags: basic, integration, time
 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.
P: 764
 Quote by Mathguy15 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?
 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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