Also known as invariant solutions. Although I think that the name similarity solutions refers mainly to one particular group: the scaling group.
Given a scalar PDE in
Suppose we have a one-parameter subgroup of symmetries of the PDE (see symmetry group of a DE system)
That is, the prolongation of it leaves invariant the hypersurface given by
The infinitesimal generator
such that
but since
This change of variables is called similarity variables, and the solutions of the original PDE obtained are called similarity solutions. See @Stephani page 172.
Particular case: travelling wave solution.
Keep an eye: A problem arises if the variable
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Author of the notes: Antonio J. Pan-Collantes
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