In the last 30 years, Approximation Theory has undergone wonderful
develop- ment, with many new theories appearing in this short interval.
This book has its origin in the wish to adequately describe this
development, in particular, to rewrite the short 1966 book of G. G.
Lorentz, "Approximation of Functions." Soon after 1980, R. A. DeVore and
Lorentz joined forces for this purpose. The outcome has been their
"Constructive Approximation" (1993), volume 303 of this series.
References to this book are given as, for example rCA, p.201]. Later,
M. v. Golitschek and Y. Makovoz joined Lorentz to produce the present
book, as a continuation of the first. Completeness has not been our
goal. In some of the theories, our exposition offers a selection of
important, representative theorems, some other cases are treated more
systematically. As in the first book, we treat only approximation of
functions of one real variable. Thus, functions of several variables,
complex approximation or interpolation are not treated, although complex
variable methods appear often.