Ancient solution
In mathematics, an ancient solution to a differential equation is a solution that can be extrapolated backwards to all past times, without singularities. That is, it is a solution "that is defined on a time interval of the form."
The term was introduced in Grigori Perelman's research on the Ricci flow, and has since been applied to other geometric flows as well as to other systems such as the Navier–Stokes equations and heat equation.
