Undefined (mathematics)

In mathematics, the word undefined has several different meanings, depending on the context.

• In geometry, simple words such as "point" and "line" are taken as undefined terms.
• In arithmetic, some arithmetic operations are called "undefined". Two of the more famous examples are division by zero and zero to the power of zero.
• In algebra, a function is said to be "undefined" at points not in its domain. For example, in the real number system, $f(x)=\sqrt{x}$ is undefined for negative $x$, i.e., no such values exist for function $f$.

Undefined terms in geometry

In ancient times, geometers attempted to define every term. For example, Euclid defined a point as "that which has no part". In modern times, mathematicians recognized that attempting to define every word inevitably led to circular definitions, and therefore left some terms, "point" for example, as undefined (see primitive notion).

Undefined operations in arithmetic

The expression 0/0 is undefined in arithmetic, as explained in division by zero (the expression is used in calculus to represent an indeterminate form).

0⁰ is often left undefined, see zero to the power of zero for details.

Values for which functions are undefined

The set of numbers for which a function is defined is called the domain of the function. If a number is not in the domain of a function, the function is said to be "undefined" for that number. Two common examples are $f(x)=\frac{1}{x}$, which is undefined for $x=0$, and $f(x)=\sqrt{x}$, which is undefined (in the real number system) for negative $x$.

Notation using ↓ and ↑

In computability theory, if f is a partial function on S and a is an element of S, then this is written as f(a)↓ and is read as "f(a) is defined." ^[1]

If a is not in the domain of f, then this is written as f(a)↑ and is read as "f(a) is undefined".

The symbols of infinity

In analysis, measure theory, and other mathematical disciplines, the symbol $\infty$ is frequently used to denote an infinite pseudo-number in real analysis, along with its negative, $-\infty$. The symbol has no well-defined meaning by itself, but an expression like $\left\{a_n\right\}\rightarrow\infty$ is shorthand for a divergent sequence, which at some point is eventually larger than any given real number.

Performing standard arithmetic operations with the symbols $\pm\infty$ is undefined. Some extensions, though, define the following conventions of addition and multiplication:

• $x+\infty=\infty$   $\forall x\in\mathbb{R}\cup\{\infty\}$.
• $-\infty+x=-\infty$   $\forall x\in\mathbb{R}\cup\{-\infty\}$.
• $x\cdot\infty=\infty$   $\forall x\in\mathbb{R}^{+}$.

No sensible extension of addition and multiplication with $\infty$ exist in the following cases:

• $\infty-\infty$
• $0\cdot\infty$ (although in measure theory, this is often defined as $0$)
• $\frac{\infty}{\infty}$

See extended real number line for more information.

Singularities in complex analysis

In complex analysis, a point $z\in\mathbb{C}$ where a holomorphic function is undefined is called a singularity. One distinguishes between removable singularities (the function can be extended holomorphically to $z$, poles (the function can be extended meromorphically to $z$), and essential singularities, where no meromorphic extension to $z$ exists.

References

1. Enderton, Herbert B. Computability: An Introduction to Recursion Theory. Elseveier, 2011, pp. 3-6, ISBN 978-0-12-384958-8

• James R. Smart, Modern Geometries Third Edition, Brooks/Cole, 1988, ISBN 0-534-08310-2