Change of dependent variable
= Change of dependent variable
A change of dependent variable expresses the unknown <function> of a <differential equation> in terms of a new unknown, while keeping the independent variable fixed. For example, substituting $y=e^{-ax/2}v$ in $y''+ay'+by=f$ removes the first <derivative> and gives $v''+(b-a^2/4)v=e^{ax/2}f$ when $a,b$ are constant. Transforming the <boundary conditions> as well is essential, particularly for a <Robin boundary condition>.