Violating intial conditions: ODEs

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SUMMARY

This discussion addresses the manipulation of initial value problems (IVP) in ordinary differential equations (ODEs) when fractions are present in the general solution. It establishes that if the initial conditions are homogeneous, such as x(0) = 0 and dx/dt(0) = 0, multiplying the solution by a nonzero constant does not violate the initial conditions. However, for non-homogeneous conditions like x(0) = 1, one must also transform the initial conditions accordingly. The example provided illustrates how to transform the ODE and initial conditions when multiplying the solution by a constant.

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CharlesNguyen
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Hi Everyone,

I had a quick question. If you have an IVP ODE and you solve for the general solution first and you had fractions in it, could you multiply by a number to make it "easier" (whole number, rather than involving fractions) without violating the initial conditions?

Thanks
 
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It depends on whether the initial conditions are homogeneous or not. For example, if you had something like ## x(0) = 0 ## and ## \frac{dx}{dt}(0) = 0 ##, you could multiply the solution to the ODE by any nonzero number you want, and it would still satisfy the ICs. But if you had, say ## x(0) = 1 ##, you would have a problem.
 
In general, you need to transform your initial conditions as well, for example,

[itex]y'=x[/itex], [itex]y(0)=1[/itex]

has as general solution [itex]y=\frac{1}{2}x^2 + C[/itex]
and C=1 when you use the initial condition.
You can multiply the general solution by 2 and use the transformation z=2y to get rid of the fractions:
[itex]z=x^2 + 2C[/itex]
but you also have to multiply the original ODE by 2 (because dz/dx=2dy/dx) and the initial condition to rewrite it to z:
[itex]z'=2x[/itex], [itex]z(0)=2[/itex]
 

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