Monad (nonstandard analysis)

In nonstandard analysis, a monad (also called halo[1]) [2] is the set of points infinitesimally close to a given point.

Given a hyperreal number x in R^∗, the monad of x is the set

${\displaystyle {\text{monad}}(x)=\{y\in \mathbb {R} ^{*}\mid x-y{\text{ is infinitesimal}}\}.}$

If x is finite (limited), the unique real number in the monad of x is called the standard part of x.

See also

• Infinitesimal

Notes

1. Goldblatt, Robert (1998). Lectures on the Hyperreals. Berlin: Springer. ISBN 0-387-98464-X.
2. NumberPhile

References

• H. Jerome Keisler: Foundations of Infinitesimal Calculus, available for downloading