In mathematics, the term undefined is often used to refer to an expression which is not assigned an interpretation or a value. The term can take on several different meanings depending on the context. For example:
In various branches of mathematics, certain concepts are introduced as primitive notions. As these terms are not defined in terms of other concepts, they may be refer to as "undefined terms".
A function is said to be "undefined" at points outside of its domainfor example, the real-valued function is undefined for negative .
In algebra, some arithmetic operations may not assign a meaning to certain values of its operands. In which case, the expressions involving such operands are termed "undefined".
Undefined terms
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 recognize that attempting to define every wordinevitably leads to circular definitions, and therefore leave some terms undefined. This more abstract approach allows for fruitful generalizations. In topology, a topological space may be defined as a set of points endowed with certain properties, but in the general setting, the nature of these "points" is left entirely undefined. Likewise, in category theory, a category consists of "objects" and "arrows", which are again primitive, undefined terms. This allows such abstract mathematical theories to be applied to very diverse concrete situations.
The expression 0/0 is undefined in arithmetic, as explained in division by zero. Mathematicians have different opinions as to whether 00 should be defined to equal 1, or be 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, which is undefined for, and, which is undefined for negative .
In trigonometry
In trigonometry, the functions and are undefined for all, while the functions and are undefined for all.
In computability theory, if ' is a partial function on ' and ' is an element of', then this is written as ', and is read as "f is defined." If ' is not in the domain of ', then this is written as ', and is read as " is undefined".
The symbols of infinity
In analysis, measure theory and other mathematical disciplines, the symbol is frequently used to denote an infinite pseudo-number, along with its negative,. The symbol has no well-defined meaning by itself, but an expression like 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 is undefined. Some extensions, though, define the following conventions of addition and multiplication:
.
.
.
No sensible extension of addition and multiplication with exists in the following cases:
