These papers extend some points of the theory. The reading and
understanding of these are not a prerequisite to a use of the theory.

## Consistency

This paper is about model theory and shows that the addition of the
specific axioms do not enable to deduce any contradiction within set
theory (unless it was already there).

## External objects

The introduction of extra axioms allows for the construction of strange
new objects. The ones that satisfy the usual properties of analysis are
called internal. The others are external and are avoided when studying the
traditional corpus of analysis.

An article which studies in depth the concept of external objects.

A simple article that shows how strange external functions can look!

## From set theory to properties of numbers

Utrasmall numbers are a consequence of extra axioms but, like all
axioms of set theory, they are about sets and not numbers. This paper shows
the link between properties of sets and properties of numbers.

## Bibliography of Hrbacek on nonstandard analysis