- #1
mrchris
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Homework Statement
I am a student in advanced calculus and I am having an issue with a grade I just received. the question was as follows:
If a function f:[0, 3]→ℝ is continuous use the Archimedes Riemann theorem to show that f is also integrable.
I want to take my answer to my professor because I think I deserve more credit than I was given, which was half of the points. I actually think my answer deserves full credit, especially if the points were taken away based on the only comment that the professor wrote on the test. Before I bring this to him, I wanted some honest critiques from the forum because maybe I am missing something and he already dislikes me as it is, so I would rather not get any further on his bad side if I don't really have a case. I will provide you with my exact answer, along with his 5 word comment exactly as they were written by both of us. The question up top is already word for word, so to start off I did not think I was required to use the epsilon delta definition of continuity to prove integrability. Since he did not mention that I am assuming that was not a reason for any deduction.
Homework Equations
archimedes riemann theorem and the definition of uniform continuity.
The Attempt at a Solution
this is it, word for word:
"If f is continuous over [0, 3], then by the extreme value theorem there is a partition P_n and a partition interval of [0, 3] which contains a u_k and v_k such that f(u_k)= m_k= inf of the interval, [x_(k-1), x_k], and f(v_k)=M_k= sup of [x_(k-1), x_k]. If we choose our u_k and v_k..."
Here, he circled my u_k and v_k and wrote, "but you have already chosen u_k and v_k" and he drew a line to the first place I wrote it. This is the part I don't understand. It seems clear to me that I am referring to the same u_k and v_k that I named earlier. I called them by the same name, I used the word "our" to denote that I was referring to the same u_k and v_k from earlier in the proof, and I never said any words that would suggest they were different, like for instance "take some v_k and u_k" or "choosing another v_k and u_k". This is also the only place in the proof where he wrote anything. I will now continue exactly where I left off:
"... and interval [x_(k-1), x_k] such that |M_k- m_k|= max |M_i - m_i|, then this statement is true: U(f,P_n)-L(f,P_n)= Ʃ (i=1 to n) [M_i- m_i][x_i- x_(i-1)]≤ |f(v_k)- f(u_k)|[Ʃ (i=1 to n)[x_i- x_(i-1)]= 3|f(v_k)- f(u_k)|. If we have chosen our partition interval P_n such that the limit as n→∞ of the gap(p_n)=0, since our v_k and u_k are contained in one of these intervals {v_n} and {u_n} are sequences such that limit as n→∞ of |v_n- u_n)| = 0. Since we know that a continuous function on a closed bounded interval is also uniformly continuous, limit as n→∞ of |v_n- u_n)| = 0 tells us that limit as n→∞ of |f(v_n)- f(u_n)| = 0
as well. By the inequality given earlier, the fact that the limit as n→∞ of |f(v_n)- f(u_n)| = 0 ensures that the limit as n→∞ of [U(f,P_n)-L(f,P_n)]=0 as well. thus by the archimedes riemann theorem, f is integrable."
Looking back, I guess I should have said that the limit proves there exists an archimedean sequence of partitions and hence blah blah blah, but i think this proof was pretty thorough, it is basically word for word from my textbook. Out of 20 points, I received 10. Half credit. Now I want you guys to tear me apart and explain to me how this proof is flawed.
Homework Statement
Homework Equations
The Attempt at a Solution
Homework Statement
Homework Equations
The Attempt at a Solution
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