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Conceptual Second order differential eqn question

  1. Nov 22, 2012 #1

    CAF123

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    I know that if ##Y_1## and ##Y_2## are two solutions of a nonhomogeneous second order differential eqn, then ##Y_1 - Y_2## is also a solution. So this motivates the following: if we set ##Y_1 = y(x)##, where ##y(x) ## is an arbritary soln of the nonhomogeneous ODE and ##Y_2 = y_p(x)##, some particular soln, we get that ##Y_1 - Y_2## is a solution to the corresponding complementary equation, $$ a(Y_1 - Y_2)'' + b(Y_1 - Y_2)' + c(Y_1 - Y_2) = 0,$$ ie ##y_c(x) = Y_1 - Y_2 = y(x) - y_p(x)##.
    I have two questions:
    1)Why set ##Y_1 = y(x)## and ##Y_2 = y_p(x)##? Could we have set ##Y_1 = y_p(x) ##and ##Y_2 = y(x)## so that in the end we get ##y(x) = y_p(x) - y_c(x),## where to recover the usual ##y(x)= y_p(x) +y_c(x),## we introduce an arbritary negative for the constants in the ##y_c(x)## term?

    2) I often read questions: Find the general soln of ... and given the initial conditions...find the particular soln. I can do these questions fine. Conceptually though and understanding what is going on, I get a little confused here because we have already defined the term 'particular soln' (as above ##y_p(x)##) in order to find the general soln. So is this two different things with the same term attached to them? I recall that ##y_p(x)## is sometimes called the particular integral?
     
    Last edited: Nov 22, 2012
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  3. Nov 22, 2012 #2

    HallsofIvy

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    NO! You don't know that- it is not true. If [itex]Y_1[/itex] and [itex]Y_2[/itex] are two solutions of the same nonhomogeneous differential equation, then [itex]Y_1- Y_2[/itex] is a solution to the associated non-homogeneous equation.

    Okay, now that is true- and is not what you said above.

    I don't see any difference. You are just changing which function you call [itex]Y_1[/itex] and which you call [itex]Y_2[/itex].

    Yes, "particular integral" is a better term than "particular solution".
     
  4. Nov 22, 2012 #3

    CAF123

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    Should that be the 'associated homogeneous' eqn?


    Changing the order will mean that the general solution is the complementary function - the particular integral rather than the complementary function + the particular integral, no?(since it is strictly ##Y_1 - Y_2##)

    Ok, thanks. So ##y_p(x)## should really be called particular integral so as to avoid confusion. Where did the 'integral' come from in its name?
     
  5. Nov 24, 2012 #4

    CAF123

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    Can you help?
     
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