- #1
wany
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Homework Statement
Find formulas for the upper and lower sums of [itex]f[/itex] on [itex]P_n[/itex], and use them to compute the value of [itex]\int_0^1f(x)dx[/itex].
[itex]P_n:=\{\frac{j}{n}:j=0,1,...,n\}[/itex] (a partition of [0,1])
[itex]\[
f(x) = \left\{ \begin{array}{ccc} 0 & 0 \le x < 1/2 \\ 1 & 1/2 \le x \le 1 \end{array} \right. \][/itex]
Homework Equations
[itex]U(f,P)=\sum\limits_{j=1}^{n} M_j(f)\Delta x_j[/itex] and
[itex]L(f,P)=\sum\limits_{j=1}^{n} M_j(f)\Delta x_j[/itex]
where [itex]M_j=sup f([x_{j-1},x_j])[/itex] and [itex]m_j=inf f([x_{j-1},x_j])[/itex]
if [itex]\lim_{n \rightarrow \infty} L(f,P_n)=\lim_{n \rightarrow \infty} U(f,P_n)[/itex] then this equals [itex]\int_0^1f(x)dx[/itex]
The Attempt at a Solution
So it is easy to see that this function is bounded on [0,1]. So now we can break this up into the different partitions, but now is where I run into a problem. It is finding the inf and the sup of each interval:
so obviously if both [itex]x_j, x_{j-1}[/itex] are < 1/2 then both inf and sup are 0;
if both [itex]x_j, x_{j-1}[/itex] are >= 1/2 then both inf and sup are 1;
so now it is possible for one case to be [itex]x_j \ge 1/2, x_{j-1} < 1/2[/itex]
in which case sup =1/2 and inf =0.
I am stuck from this point. Any help would be appreciated.