Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fourier Series of sin (pi*t)

  1. Apr 23, 2004 #1
    I am having trouble finding the An and Bn coefficients for the fourier series f(t) = sin (pi*t) from 0<t<1, period 1

    Please help! Thank you!
  2. jcsd
  3. May 29, 2004 #2
    Newbie wants to bump this query:

    I was having trouble with this problem as well until I discovered a simple algebra error; here is how I set it up

    [tex]a_0 = \frac{1}{\frac{1}{2}}\int_0^1 Sin(\pi t) \;dt = \frac{4}{\pi}[/tex]

    [tex]A_N = \frac{1}{\frac{1}{2}}\int_0^1 Sin(\pi t) Cos(2 n \pi t)\; dt [/tex]

    [tex]B_N = \frac{1}{\frac{1}{2}}\int_0^1 Sin(\pi t) Sin (2 n \pi t)\; dt [/tex]

    A source advises the Trigonometric Identities:

    [tex]2 Sin[A] Cos = Sin [A+B] + Sin[A-B] [/tex]

    [tex]2 Sin[A] Sin = Cos[A-B] - Cos[A+B] [/tex]

    let [tex] u = \pi t + 2 n\pi t = A + B \mbox{\quad and let\quad} v = \pi t - 2 n \pi t = A -B[/tex]


    [tex]A_n = 2\; \frac{1}{2}\;\;\frac{ 1}{\pi (1+ 2 n)} \;\;\int_0^{\;\pi(1+2 n)} Sin(u)\; du[/tex]

    [tex] \quad\quad\quad+ \quad 2\;\frac{1}{2}\;\;\frac{ 1}{\pi (1- 2 n)} \;\;\int_0^ {\;\pi(1-2 n)} Sin(v)\;dv [/tex]

    [tex]B_n = 2\; \frac{1}{2}\;\;\frac{ 1}{\pi (1-2 n)} \;\;\int_0^{\;\pi(1-2 n)} Cos(v)\; dv[/tex]
    [tex]\quad\quad + \quad 2 \;\frac{1}{2}\;\;\frac{ 1}{\pi (1+ 2 n)}\;\; \int_0^ {\;\pi(1+2 n)} Cos(u)\;du [/tex]

    where the substituted limits of integration, [tex] \pi (1 \pm 2n)=\pi\pm2n\pi [/tex] for [tex] \mbox{integer}\; n\geq1[/tex] are equivalent to [tex]\pi[/tex] due to periodicity of the Sine and Cosine functions.
    Last edited: Jun 28, 2004
  4. May 30, 2004 #3


    User Avatar
    Science Advisor

    So, basically, you need to integrate sin([pi]t)sin(nt) and sin([pi]t)cos(nt).

    Use the trig identities sin(a)sin(b)= (1/2)(cos(a-b)- cos(a+b)) and
    sin(a)sin(b)= (1/2)(sin(a+b)+ sin(a-b)).
  5. May 30, 2004 #4
    Yes, except for typo--second equation should read [tex]sin(a)cos(b) = 1/2 (sin[a+b] + sin[a-b])[/tex]and with the substitutions suggested these integrals tidy up rather nicely [itex]\mbox{ (hint---} B_n = 0\mbox{ for all integers,\:} n\geq 1)[/itex].
    I get a good form for the actual function [itex]sin(\pi t) [/itex] being modelled by Fourier using
    [tex] \frac{a_0}{2}+\sum_1^{\infty} A_n\;Cos(2 n \pi t) \quad\mbox{ \quad n is an integer}[/tex]
    [tex]A_n =\frac{-4}{\pi (4n^2-1)}[/tex]
    [tex]f(t)=\frac{2}{\pi}-\frac{4}{\pi}\sum_1^{\infty}\frac{Cos(2n\pi t)}{4n^2-1}=Sin(\pi t)[/tex]

    [tex]f(t)\sim\frac{2}{\pi}\;-\;\frac{4}{\pi}\left(\frac{Cos(2\pi t)}{3}\;+\;\frac{Cos(4\pi t)}{15}\;+\;\frac{Cos(6\pi t)}{35}\;+\;\cdots\right ) [/tex]
    Last edited: Jun 24, 2004
  6. Jun 9, 2004 #5
    Follows an attempt to display a plot of [itex]Sin(\pi t)[/tex] of period 1 using the initial constant term and 6 iterations of the Cos term. Click on the bitmap file.

    Attached Files:

    • foo.bmp
      File size:
      49.8 KB
    Last edited: Jun 10, 2004
  7. Jun 9, 2004 #6
    You guys are missing the forest for the trees!

    The fourier series is a sum of sines and cosines, and it's unique. So if you can find one set of coefficients that works, you've got THE fourier series.

    So if

    sin(pi*t) = a0 + An*SUM[sin(n*pi*t)] + Bn*SUM[cos(n*pi*t)],

    can't you just look at that and see a set of coefficients that will make the left and right hand sides of that equation the same?
    Last edited: Jun 9, 2004
  8. Jun 9, 2004 #7
    OK, here is the same problem except make the period 2 and let the function to be modelled by Fourier series be:

    [itex] f(t) =\left\{\begin{array}{cr}0&\mbox{if\;\;}-1<t<0\\sin(\pi t)&\mbox{if\quad} 0<t<1\end{array}\right [/itex]

    Calculations very similar to those pictured earlier above and adjusted only slightly for the period size being doubled lead to the following series

    [itex]\frac{1}{\pi} \;+\;\frac{1}{2}\;Sin(\n\pi t)\;-\frac{2}{\pi}\;\sum_{n=2}^{\infty}\;\frac{Cos(n\;\pi\;t)}{n^2-1}=f(t)[/itex]

    This series produces a very nice periodic plot modelling f(t) with only 5 iterations of the Cos term .[tex]B_1=\frac{1}{2}[/tex] was calculated separately with its own integral as the general expression for [tex]B_n[/tex] is undefined at n=1 and 0 for [tex]n\geq2[/tex]. For the same reason the Cos terms are iterated starting at n=2 with [tex]A_1 = 0[/tex])

    Click on the bitmap to see the plot so generated.

    Attached Files:

    • foo2.bmp
      File size:
      49.8 KB
    Last edited: Jun 11, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook