How Can You Effectively Change Variables to Solve a Specific PDE?

[jimbo007]

hi,
i am having difficulty trying to find a change of variables to solve this partial differential equation
\frac{\partial f}{\partial t} = t^\gamma \frac{\partial ^2 f}{\partial x^2}
not sure how to pluck out a change of variables by looking at the equation as its definitely not obvious to the untrained eye (me).
have tried
f(x,t) = p(\eta) (not sure if i am even on the right track doing this)
where
\eta = \frac{x}{t^{\gamma}}
but that just gets me in a giant mess with
-\gamma\frac{x}{t}\frac{\partial p}{\partial \eta} = \frac{\partial ^2 p}{\partial \eta ^2}
i may have done \frac{\partial ^2 f}{\partial x^2}= \frac{\partial \eta}{\partial x}\frac{\partial ^2 p}{\partial \eta ^2}\frac{\partial \eta}{\partial x} wrongly.

any suggestions?
thanks for the time

 

[topsquark]

hi,
i am having difficulty trying to find a change of variables to solve this partial differential equation
\frac{\partial f}{\partial t} = t^\gamma \frac{\partial ^2 f}{\partial x^2}
not sure how to pluck out a change of variables by looking at the equation as its definitely not obvious to the untrained eye (me).
have tried
f(x,t) = p(\eta) (not sure if i am even on the right track doing this)
where
\eta = \frac{x}{t^{\gamma}}
but that just gets me in a giant mess with
-\gamma\frac{x}{t}\frac{\partial p}{\partial \eta} = \frac{\partial ^2 p}{\partial \eta ^2}
i may have done \frac{\partial ^2 f}{\partial x^2}= \frac{\partial \eta}{\partial x}\frac{\partial ^2 p}{\partial \eta ^2}\frac{\partial \eta}{\partial x} wrongly.

any suggestions?
thanks for the time

I don't know about a substitution, but if you're just after a solution you can set f(x,t) = X(x)T(t) and the equation becomes separable.

-Dan

 

[HallsofIvy]

There is no need to get "x" involved in your change of variable. You want to go from
\frac{\partial f}{\partial t} = t^\gamma \frac{\partial ^2 f}{\partial x^2}
or, similarly,
t^{-\gamma}\frac{\partial f}{\partial t} = \frac{\partial ^2 f}{\partial x^2}
to something like
\frac{\partial f}{\partial \eta} = \frac{\partial ^2 f}{\partial x^2}
which means you want
\frac{\partial f}{\partial \eta}= \frac{\partial f}{\partial t}\frac{dt}{d \eta}= t^{-\gamma}\frac{\partial f}{\partial t}
So you must have
\frac{dt}{d\eta}= t^{-\gamma}
or
\frac{d\eta}{dt}= t^{\gamma}