I am starting a series of “What Is” posts. These will give brief descriptions of important concepts in numerical analysis and related areas, with a focus on topics that arise in my research. I will try to adhere to the following criteria.

- The posts are no more than two or three screenfuls (about 500 words).
- The posts contain a minimum of mathematical symbols, equations, and citations.
- The posts are accessible to upper level undergraduate students in any mathematically oriented subject.
- The list of references given for further reading is very small and concentrates on those that are broad, readable, and contain further references to the literature.

I will make PDF versions (from ) of the posts available on GitHub in the repository What-Is. These are much better for printing. They contain hyperlinks, which will be clickable in the PDF but not visible in a printout.

I will be happy to receive suggestions for topics to cover.

From 2002–2019 the Notices of the American Mathematical Society published a “What Is …” series of articles, latterly in the Graduate Student section. An index of the articles is available here. Those articles are almost exclusively on pure mathematics and I don’t anticipate much, if any, overlap with my series.