- #1
- 2,076
- 140
Homework Statement
Suppose that f is a bounded, increasing function on [a,b]. If p is the partition of [a,b] into n equal sub intervals, compute Sp - sp and hence show f is integrable on [a,b]. What can you say about a decreasing function?
Homework Equations
We partition [a,b] into sub-intervals.
For each i, we let : [itex]m_i = inf \left\{{f(x)|x_{i-1} ≤ x ≤ x_i}\right\}[/itex] and [itex]M_i = sup \left\{{f(x)|x_{i-1} ≤ x ≤ x_i}\right\}[/itex]
Now, we define [itex]s_p = \sum_{i=1}^{n} m_i Δx_i[/itex] as the lower sum and [itex]S_p = \sum_{i=1}^{n} M_i Δx_i[/itex] as the upper sum.
Some more info in my notes :
Let [itex]M = sup \left\{{f(x)|x \in [a,b]}\right\}[/itex] and [itex]m = inf \left\{{f(x)|x \in [a,b]}\right\}[/itex]. Then we get sp ≤ M(b-a) so the set of all possible sp is bounded above.
Let I = sup{sp} and J = inf{Sp}
Definition : if I = J, then f(x) is integrable.
Now another theorem I could use : For a bounded function f on [a,b]. f is integrable if and only if :
[itex]\forall ε>0[/itex] there is a partition p of [a,b] such that Sp < sp + ε.
The Attempt at a Solution
So we are given that f is a bounded increasing function. This means that m ≤ f ≤ M for some lower bound m and upper bound M. For any partition p of [a,b] into n sub-intervals, we also have :
[itex]m_i = inf \left\{{f(x)|x_{i-1} ≤ x ≤ x_i}\right\}[/itex] and [itex]M_i = sup \left\{{f(x)|x_{i-1} ≤ x ≤ x_i}\right\}[/itex]
Now before I jump any further I want to confirm the direction I'm going in. I have two theorems I provided and I'm wondering which one is more appropriate to use here.
Last edited: