## Approximation theory

"The fundamental problem of approximation theory is to resolve a possibly complicated function, called the *target function*, by simpler, easier to compute functions called the *approximants*. Increasing the resolution of the target function can generally only be achieved by increasing the complexity of the approximants. The understanding of this trade-off between resolution and complexity is the main goal of constructive approximation."

R. A. DeVore, Nonlinear approximation.