arXiv:2610.12888v1 Announce Type: new Abstract: This is a lecture note to introduce the innovative concept of numerical smoothness and its application in the error analysis of numerical solutions of evolution equations. We start from the two fundamentally different error splittings. One splitting is popular, which brings us to local error defined along the original solution and error propagation by numerical schemes. The well know Lax Equivalence Theorem is based on this error splitting. The other splitting can only be seen in a few numerical ODE books, which brings us to local error defined along the numerical solution and error propagation by the solutions of the evolution equations. The innovative concept of numerical smoothness arises from the second error splitting. While there are many difficulties when we use numerical stability in error analysis of nonlinear evolution equations, we will show that numerical smoothness works naturally in error analysis/estimation/control with advantages in several ways. We will define temporal and spatial numerical smoothness indicators, prove the necessity of the boundedness of these indicators for having optimal convergence rate, and show computable error estimates as functions of these smoothness indicators. Moreover, these error estimates are optimal in the approximatons' convergence rates and also optimal in the error propagation of the evolution equation. In comparison with error analysis framework of the Lax Equivalence Theorem, from the necessity results and the error estimates proven in the numerical smoothness framework, we have a new equivalence theory: Consistency + Numerical Smoothness <=> optimal error estimates. Unlike the Lax Equivalence Theorem, the new equivalence theory works well for nonlinear problems, nonlinear schemes, and many other complications of problems.