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

I Question about second order linear differential equations

  1. Aug 21, 2017 #1
    Hi everybody.

    I need to learn how to solve this kind of equation by decomposing y in a serie of functions. All the examples I have seen are of homogeneous functions. I would be extremely thankfull if someone pointed me to some text in which this is done-explained.

    Thanks for reading.
  2. jcsd
  3. Aug 21, 2017 #2


    Staff: Mentor

    Here are two first page entries of a Google search:
    http://www.math.psu.edu/tseng/class/Math251/Notes-2nd order ODE pt2.pdf (Pennsylvania)
    http://tutorial.math.lamar.edu/Classes/DE/NonhomogeneousDE.aspx (Texas)
    Also a good starting point is Wikipedia and its links there:
    (Depending on the languages you speak or understand, it could be also worth it to change between different languages on Wikipedia as they are not 1:1 translations but individual and thus different descriptions of the same topic.)
  4. Aug 21, 2017 #3


    Staff: Mentor

    Are you referring to separation of variables when you say y is composed of series of functions?

    If so then separation of variables is actually for a product of functions?


    So in addition to what @fresh_42 has provided may I suggest:


    It's a website that describes many of these ODE and PDE solution recipes in short videos on each type of problem:


    You can view them in any order to bolster your understanding of a given solution recipe.
    Last edited: Aug 22, 2017
  5. Aug 22, 2017 #4


    Staff: Mentor

    I don't think he is, but I'm not certain of that.

    By "series of functions" I believe he means a sum of two functions. In a homogeneous 2nd order diff. equation, the standard approach is to assume solutions of the form ##y = e^{rx}##. Finding the roots of the characteristic equation for the diff. equation gets you the values of r.

    For example, with the differential equation y'' + 5y' + 4y = 0, the characteristic equation is ##r^2 + 5r + 4 = 0##, the roots of which are r = -4 and r = -1. The general solution would then be ##y = c_1e^{-4t} + c_2e^{-t}##. If initial conditions are given, the constants ##c_1## and ##c_2## can be determined.
  6. Aug 22, 2017 #5


    Staff: Mentor

    Any textbook on differential equations would have a section on this. A textbook that comes to mind is one by Boyce and DiPrima, but there are lots more.
    Last edited: Aug 23, 2017
  7. Aug 22, 2017 #6


    User Avatar
    Science Advisor
    Homework Helper

  8. Aug 23, 2017 #7


    Staff: Mentor

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted