ODE types reductible to the variables separable case
There are certain of non-linear ordinary differential equations of first order (http://planetmath.org/ODE) which may by a suitable substitution be to a form where one can separate (http://planetmath.org/SeparationOfVariables) the variables.
I. So-called homogeneous differential equation
This means the equation of the form
where and are two homogeneous functions of the same degree (http://planetmath.org/HomogeneousFunction). Therefore, if the equation is written as
its right hand side is a homogeneous function of degree 0, i.e. it depends only on the ratio , and has thus the form
(1) |
Accordingly, if this ratio is constant, then also is constant; thus all lines constant are isoclines of the family of the integral curves which intersect any such line isogonally.
We can infer as well, that if one integral curve is represented by , , then also , an integral curve for any constant . Hence the integral curves are homothetic with respect to the origin; therefore some people call the equation (1) a similarity equation.
For generally solving the equation (1), make the substitution
The equation takes the form
(2) |
which shows that any root (http://planetmath.org/Equation) of the equality gives a singular solution . The variables in (2) may be :
Thus one obtains , whence the general solution of the homogeneous differential equation (1) is in a parametric form
II. Equation of the form y= f(ax+by+c)
It’s a question of the equation
(3) |
where , and are given constants. If is constant, then is constant, and we see that the lines constant are isoclines of the intgral curves of (3).
Let
(4) |
be a new variable. It changes the equation (3) to
(5) |
Here, one can see that the real zeros of the right hand side yield lines (4) which are integral curves of (3), and thus we have singular solutions. Moreover, one can separate the variables in (5) and integrate, obtaining as a function of . Using still (4) gives also . The general solution is
Example. In the nonlinear equation
which is of the type II, one cannot separate the variables and . The substitution converts it to
where one can separate the variables. Since the right hand side has the zeros , the given equation has the singular solutions given by . Separating the variables and , one obtains
whence
Accordingly, the given differential equation has the parametric solution
References
- 1 E. Lindelöf: Differentiali- ja integralilasku III 1. Mercatorin Kirjapaino Osakeyhtiö, Helsinki (1935).
Title | ODE types reductible to the variables separable case |
Canonical name | ODETypesReductibleToTheVariablesSeparableCase |
Date of creation | 2013-03-22 18:06:36 |
Last modified on | 2013-03-22 18:06:36 |
Owner | pahio (2872) |
Last modified by | pahio (2872) |
Numerical id | 13 |
Author | pahio (2872) |
Entry type | Topic |
Classification | msc 34A09 |
Classification | msc 34A05 |
Related topic | SeparationOfVariables |
Related topic | ODETypesSolvableByTwoQuadratures |
Related topic | TheoryForSeparationOfVariables |
Defines | homogeneous differential equation |
Defines | similarity equation |