Undefined (mathematics)

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

in geometry, simple words such as "point" and "line" are taken as undefined terms or primitive notions;
in arithmetic, some arithmetic operations are called "undefined", such as 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 $f$ .

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).

POINT (an undefined term) In geometry, a point has no dimension (actual size). Even though we represent a point with a dot, the point has no length, width, or thickness. Our dot can be very tiny or very large and it still represents a point. A point is usually named with a capital letter. In the coordinate plane, a point is named by an ordered pair, (x,y).

LINE (an undefined term) In geometry, a line has no thickness but its length extends in one dimension and goes on forever in both directions. Unless otherwise stated a line is drawn as a straight line with two arrowheads indicating that the line extends without end in both directions. A line is named by a single lowercase letter, , or by any two points on the line, .

PLANE (an undefined term) In geometry, a plane has no thickness but extends indefinitely in all directions. Planes are usually represented by a shape that looks like a tabletop or a parallelogram. Even though the diagram of a plane has edges, you must remember that the plane has no boundaries. A plane is named by a single letter (plane m) or by three non-collinear points (plane ABC).

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 $\infty$ exists 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

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