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GLOSSARY  

 

Posted 2 January 2009

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

I

¨  I (bold) may denote the unit interval, the set of real numbers x for which .  It may however denote any bounded interval of real numbers.

¨  I may be used as the name of an arbitrary index set.

¨  For some authors, I (bold) or  (blackboard bold)  means the set of integers; however, Z or  is also used for this.

¨  I may denote the identity function.  “id” is also used for this. 

¨  I may denote the identity matrix for a dimension given by context.

¨  i  (usually lowercase) denotes one of the square roots of 1.  (The other one is then i).  In engineering, j is very frequently used instead of i for this. 

These various meanings are quite commonly used without explanation.

 

i.e.

The expression “i.e.” means that is.  This abbreviation is confused by many readers and writers (including writers in the research literature) with “e.g.”, which means “for example”.  This is a very significant difference in meaning.   Consider

¨  “Let n be an even integer, e.g. 6.”   Good

¨  “Let n be an even integer, i.e. 6.”     Bad.  It suggests that 6 is the only even integer.

¨  “Let n be an even integer, e.g. divisible by 2.”  Bad.  It says, “Let n be an even integer, for example divisible by 2.”  The reader expects the phrase after the comma to provide a particular integer that is even, but it doesn’t.    

¨  “Let n be an even integer, i.e. divisible by 2.”   Good

 

If you desperately want to be understood, you should never use “i.e.” and “e.g.”

Too many readers don’t know what they mean.

identify

To identify an object A with another object B is to regard them as the same object.  In particular each point in A is regarded as the same as a particular point in B.  In most cases, the author has in mind a particular way of matching the points in A and the points in B.  This “particular way” may be presented informally or as a formal mathematical construction.

Examples

¨  You may regard each point of the real plane as having specific coordinates, an ordered pair of real numbers.  (“Consider the point with coordinates (2.3, -1.45).”)  On the other hand, you may think of the points of the real plane as identified with ordered pairs of real numbers.  (“Consider the point (2.3, -1.45).”)  These are two ways of thinking about the plane (an example of conceptual blending).  Both Wikipedia and MathWorld define the plane in such a way that its points are represented by pairs of real numbers.  That is desirable in a definition since it looks forward to the generalization to manifolds.  But you can think about the plane either way and get away with it.

¨  The Möbius strip may be constructed by identifying or gluing the edge

 


of the unit square with the edge

 

in such a way that (x, 0) is identified with (1  x, 1). This can be thought of as saying that the coordinates  (x, 0) and (1  x, 1) refer to the same point.  This is an example of identifying objects by a formal mathematical construction.

Usage

In ordinary English, “identify” means to give a name to. This presumably could cause cognitive dissonance.

identity

This word has three common meanings.

Equation that always holds

An identity in this sense is an equation that holds between two expressions for any valid values of the variables in the expressions. Thus, for real numbers (in fact for complex numbers), the equation

                                                     

is an identity.

Fine point

Consider the assertion  “If f(x) is a constant function, then for any x ”.  The equation  is conditional on the hypothesis “f(x) is a constant function” being true.  In this situation, the equation  would not be called an identity.  This requires further lexicographical investigation.

Usage

Sometimes in the case of an identity the symbol is used instead of the equals sign.


Identity element of an algebraic structure

If  for all elements x in an algebraic structure with binary operation , then e is an identity or identity element for the structure. Such an element is also called a unit element or unity.   

A ring with identity may mean a ring with identity element, but it can also mean a ring subject to an algebraic identity.

Identity function

For a given set S, the function from S to S that takes every element of S to itself is called the identity function.  (More about that here.) This is an example of a polymorphic definition. 

 

The identity function on S is the identity element of the algebraic structure consisting of all functions from a set S to itself with composition as operation.

if

In conditional assertions

The ways if is used in stating conditional assertions is discussed here.  See also let, which covers assume and suppose as well.

In definitions

It is a convention that the word if used to introduce the definiens in a definition means “if and only if”.  

Examples

a:  “An integer is even if it is divisible by 2.”

Some authors regularly use if and only if or “iff” in definition, so the definition above would be given as

b:  “An integer is even if and only if it is divisible by 2.”

But note:  The following statement is not a definition, but an equivalence:

        c:  “An integer is divisible by 2 if and only if it is divisible by 2.”

Even though “if and only if” is used in both sentences, they don’t have the same status.   In statement b the left side and the right side are not symmetric.  The point of statement b is to give a meaning to the left side.  Statement c is symmetric; you could just as correctly say

              “An integer is divisible by 2 if and only if it is divisible by 2.”

In the precondition of a definition

If can be used in the precondition of a definition to introduce some requirements on an object mentioned in the definition.  For example,  definition a above could have been worded this way:

“If n is an integer, then it is even if it is divisible by  2.”

if and only if

If P and Q are two assertions, the statement “P if and only if Q” is an assertion that P and Q  are equivalent.

This statement may be abbreviated “P iff Q”. 

image

See the article in Wikipedia.

imply

If P and Q are assertions, “P implies Q” means exactly the same thing as “If P, then Q”.  This is discussed in detail under conditional assertions.

in

In is used in mathematical discourse in all its English meanings, as well as in some meanings that are peculiar to mathematics.

¨  A is in B” can mean  

¨  A is in B” can mean .

¨  You may say “A is in B” when A is an equation whose solution set is included in B, or a geometric figure whose points are included in B. Example:

"The unit circle  is in the Euclidean plane."

¨  You may say “A is in B” when A is a subexpression of B.   For example, x is a variable in , and  is a term in .

¨  A is P in B, where P is a property, may mean that A has property P with respect to B, where B is a constituent of A or a related structure (for example a containing structure). Thus in group theory you can say  A is normal in B, where A is a subgroup of the group B. As another example,  is differentiable in y (and x).

¨  You may describe an intersection using "in". For example, the sets {1, 2, 3, 4} and {1, 3, 5, 7} intersect in {1, 3} (or intersect in 1 and 3.)

in general

¨  Statements such as “In general, P” or “P is true in general” mean that P is true no matter what values you substitute for its variables.

¨  Statements such as “In general, not P” mean that P is not necessarily true.”

Examples

¨  “The equation  is true in general”  means that for all x,  is true. 

¨  “In general, not every subgroup of a group is normal in the group”  means that an arbitrary subgroup of a group need not be normal in the group.  The statement specifically implies that there are examples where the subgroup is not normal in the group.

Remark

In most circumstances, you can substitute generally for “in general”.

increasing

Let  be a function. 

¨  f is increasing if for all x and y, if x < y then f(x) < f(y).

¨  f is decreasing if for all x and y, if x < y then f(x) > f(y).

¨  f is nonincreasing if for all x and y, if x < y then .

¨  f is nondecreasing if for all x and y, if x < y then .

These words can be used for functions from and to other ordered sets, as well.

They are also used for sequences, since a sequence is a function on its index set.   For example, a sequence  is increasing if for all i and j in its index set, .


inequality

 An inequality is an assertion of the form , where s and t are terms and α is one of the relations <, >,  or .  An assertion of the form  is not called an inequality.

 

inhabit

The phrase “A inhabits B” can mean any of several things:

¨  A is an element of B.

¨  A is an entry in the list B.

¨  A is an expression in the delimiters B.  (For example, “The function inhabiting the curly braces is increasing.”)

Lives in is used similarly in conversation, but I have found only a few citations in print.

injective

Functions can have a property called injective, and so can modules.  These two uses are unrelated.

integral

This word has three different meanings.

Being an integer

Integral is used as an adjective to require that the noun phrase it modifies denote an integer.  For example, 8 is an integral power of 2 but 9 is not.  This is also used for the more general notion of algebraic integer (MW, Wi).

Antiderivative

An integral of a function is an antiderivative of the function.  It may be called the indefinite integral to distinguish it from the definite integral below.  The indefinite integral of f may be denoted by  which determines the integral only up to a constant.

You may also refer to a specific antiderivative using the form .

The word integral is also used to denote a solution of a more general differential equation.

Definite integral

Integral is also used to denote a definite integral: this operator takes an integrable function and an interval (or more general space) on which the function is defined and produces a number. The definite integral of f on an interval [a, b] is denoted  or .  

intuition

Mathematicians may use the word intuitively in referring to some image or metaphor they have about a type of object. 

Examples

“Intuitively, the squaring function is a machine that turns, for example, 3 into 9.”

They may admit the intuition but point out its weaknesses:

“Intuitively, an open set in the plane is every point in a region not including its boundary.  In fact, not all open sets fit this description, for example…” 

is

See be.