Dirichlet Kernel
Not what you're looking for?
Let D_n (theta) = sum(k=-N to N) e^ik(theta)= sin ((N+1/2)theta)/sin(theta/2) and define L_n = 1/2Pi integral (from - Pi to Pi) |D_n (theta)| d(theta)
prove that L_N is greater than or equal to c log (N) for some constant c>0
Hint: show that |D_n(theta)| is greater than or equal to c sin ((n+1/2)theta)/|theta| change variables and prove that
L_n greater than or equal to c intergral (from Pi to N Pi) |sin(theta)|/|theta| d(theta) +O(1). Write the integral as sum (k=1 to N-1) integral (k pi to (k+1) Pi. To conclude use the fact that sum (k=1 to n) 1/k is greater than or equal to c log n. A more careful estimate gives L_n =(4/Pi^2) log N + O(1).
This is problem 2a on this page.
http://books.google.ca/books?id=FAOc24bTfGkC&pg=PA66&lpg=PA66&dq=Dirichlet+Kernel+clogN&source=bl&ots=g-UUMpfAeC&sig=lTuZwtgM8Q-U3V6-_xLSeFKHgg8&hl=en&ei=YVXHTO61OpO4sQP2-_2RDQ&sa=X&oi=book_result&ct=result&resnum=6&ved=0CDIQ6AEwBQ#v=onepage&q=Dirichlet%20Kernel%20clogN&f=false
Purchase this Solution
Solution Summary
Complete the proof.
Purchase this Solution
Free BrainMass Quizzes
Graphs and Functions
This quiz helps you easily identify a function and test your understanding of ranges, domains , function inverses and transformations.
Solving quadratic inequalities
This quiz test you on how well you are familiar with solving quadratic inequalities.
Geometry - Real Life Application Problems
Understanding of how geometry applies to in real-world contexts
Multiplying Complex Numbers
This is a short quiz to check your understanding of multiplication of complex numbers in rectangular form.
Exponential Expressions
In this quiz, you will have a chance to practice basic terminology of exponential expressions and how to evaluate them.