An evolution problem is locally well posed near a datum u0u_0 in a data space XX if there are a time T>t0T>t_0, a neighborhood VXV\subseteq X of u0u_0, and a specified solution space YTY_T such that:

  1. Every datum in VV has a solution on [t0,T][t_0,T].
  2. The solution is unique in the asserted class.
  3. The map from the datum to its solution in YTY_T is .

The equation, coefficients, and forcing are held fixed unless the data space explicitly includes them.

Dependence on the chosen spaces

Well-posedness is a statement about both an equation and its data and solution topologies. Existence alone is insufficient. Uniqueness in a smooth class also does not automatically imply uniqueness in every larger weak-solution class.

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