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Does Homogeneous = Separable? 
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#1
Sep1204, 12:47 PM

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PF Gold
P: 5,196

Hi,
Please bear with me, I've only had the first sort of "pseudolecture" in ordinary d.e.'s this past week, and I was doing some reading ahead. It occurred to me that if linear firstorder differential equations are those that can be written in the general form: [tex] \frac{dy}{dx} + P(x)y = Q(x) [/tex] and if I understood my prof's remark correctly that homogeneous linear firstorder d.e.'s are those for which [itex] Q(x) = 0 [/itex], then all homogeneous firstorder linear differential equations are actually separable because: [tex] \frac{dy}{dx} + P(x)y = 0 [/tex] [tex] \frac{dy}{dx} = P(x)y [/tex] Which can be solved as follows: [tex] \int{\frac{dy}{y}} = \int{P(x)dx} [/tex] ^^Looks separable to me. I just wanted to make sure I was getting the basics before I moved on. So is the thread title statement true then? Oh yeah, and I'm guessing that the converse is not always true right? A separable firstorder d.e. is not necessarily a homogeneous linear d.e. right? After all, [itex] \frac{dy}{dx} = \frac{6x^2}{2y + cosy} [/itex] is not linear in [itex] y [/itex] right? But it looks like it could be homogeneous nonlinear (if such a thing exists, I don't know???). In that case, would the statement homogeneous = separable be true in the most general sense? Moving on with the original example (I'm just solving while "LaTeXing" to see what comes out) ... [tex] \lny = \int{P(x)dx} [/tex] [tex] y = e^{\int {P(x)dx}} [/tex] Now, the most general solution for [itex] y [/itex] must include the most general antiderivative, so we'll have a [itex] C [/itex] stuck in there if and when we solve the integral: [tex] y = \pm e^{\int {P(x)dx} + C} = \pm e^{\int {P(x)dx}}e^C = \pm Ae^{\int {P(x)dx}} [/tex] Whoah, cool! So in a homogeneous first order d.e. the solution takes the form of some constant [itex] \pm A [/itex] times the reciprocal of the integrating factor [itex] I(x) [/itex]?! Is this always true? EDIT: Yeah actually I can see why that is. Take the usual statement about the integrating factor: [tex] (I(x)y)^{\prime} = I(x)Q(x) [/tex] If [itex] Q(x) = 0 [/itex], then [tex] (I(x)y)^{\prime} = 0 [/tex] [tex] I(x)y = \pm A [/tex] [tex] y = \frac{\pm A}{I(x)} [/tex] Awesome! I discovered a lot more writing this post than I expected. I hope someone will correct me if I've made any errors. 


#2
Sep1204, 12:53 PM

P: 695

Just wondering, but do you really need to state that the constant of integration is +/ A, instead of just A (in the part where you talk about integrating factors)?



#3
Sep1204, 01:51 PM

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PF Gold
P: 5,196

Good point. In fact, I guess I could have gotten rid of the +/ way up here:
[tex] \pm (e^{\int {P(x)dx}})(e^C) = Ae^{\int {P(x)dx}} [/tex] just combining everything into "A", which would remain as A for the ensuing discussion. 


#4
Sep1204, 05:48 PM

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Thanks
PF Gold
P: 39,565

Does Homogeneous = Separable?
In answer to the original question, yes, a first order d.e. that is "homogeneous" in this sense: P(x) dy/dx+ Q(x)y= 0 is trivially seperable: dy/y= (Q(x)/P(x))dx.
But be careful: in the limited area of FIRST ORDER d. e.s, the term "homogeneous" is often used in quite a different way (the d.e. A(x,y)dx+ B(x,y)dy= 0 is "homogeneous" if and only if B(&lamda;x,&lamda;y)/A(λx, λy)= B(x,y)/A(x,y) (essentially that means that the total exponent of x and y in each term is the same). 


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