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Thread: Can anybody explain me this fourier series page in simple steps

  1. #1
    Member moonnightingale's Avatar
    Sep 2009

    Can anybody explain me this fourier series page in simple steps

    Fourier Series for Even Functions

    For an even function f(t), defined over the range -L to L (i.e. period = 2L), we have the following handy short cut.
    f(t) is even,
    it means the integral will have value 0. (See Properties of Sine and Cosine Graphs.)
    So for the Fourier Series for an even function, the coefficient bn has zero value:
    bn = 0
    So we only need to calculate a0 and an when finding the Fourier Series expansion for an even function f(t):

    An even function has only cosine terms in its Fourier expansion:
    Specally how he has written ao/2 in last equation. Thanks
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  2. #2
    Super Member
    Jun 2009
    The first term is a constant and you can call it whatever you want.

    From the start point of

    $\displaystyle f(t)=C+a_{1}\cos(\pi t/L)+a_{2}\cos(2\pi t/L)+\dots$ ,

    integrating between $\displaystyle -L$ and $\displaystyle L$ produces the result

    $\displaystyle C = \frac{1}{2L}\int_{-L}^{L}f(t)\,dt.$

    If the $\displaystyle a$ coefficients are calculated (by multiplying both sides by $\displaystyle \cos(n\pi t/L)$ and integrating betwen $\displaystyle -L$ and $\displaystyle L$), you find that the fraction in front of each integral is $\displaystyle 1/L$.

    So, you have a choice. Either leave things as they are in which case one of the integrals has $\displaystyle 1/2L$ at the front and the others have $\displaystyle 1/L$. Or, call the constant $\displaystyle C=a_0/2$, in which case the 2's cancel and you have the consistency of all of the integrals having $\displaystyle 1/L$ at the front.

    Some choose the first option others choose the second.
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