# Multiplication

**Multiplication** (often denoted by the cross symbol **×**, by the mid-line dot operator **⋅**, by juxtaposition, or, on computers, by an asterisk *****) is one of the four elementary mathematical operations of arithmetic, with the other ones being addition, subtraction, and division. The result of a multiplication operation is called a product.

The multiplication of whole numbers may be thought of as a repeated addition; that is, the multiplication of two numbers is equivalent to adding as many copies of one of them, the *multiplicand*, as the quantity of the other one, the *multiplier*. Both numbers can be referred to as *factors*.

Here, 3 (the *multiplier*) and 4 (the *multiplicand*) are the *factors*, and 12 is the *product*.

One of the main properties of multiplication is the commutative property, which states in this case that adding 3 copies of 4 gives the same result as adding 4 copies of 3:

Thus the designation of multiplier and multiplicand does not affect the result of the multiplication.^{[1]}

The multiplication of integers (including negative numbers), rational numbers (fractions) and real numbers is defined by a systematic generalization of this basic definition.

Multiplication can also be visualized as counting objects arranged in a rectangle (for whole numbers), or as finding the area of a rectangle whose sides have some given lengths. The area of a rectangle does not depend on which side is measured first—a consequence of the commutative property.

The product of two measurements is a new type of measurement. For example, multiplying the lengths of the two sides of a rectangle gives its area. Such products is the subject of dimensional analysis.

The inverse operation of multiplication is division. For example, since 4 multiplied by 3 equals 12, 12 divided by 3 equals 4. Indeed, multiplication by 3, followed by division by 3, yields the original number. The division of a number other than 0 by itself equals 1.

Multiplication is also defined for other types of numbers, such as complex numbers, and more abstract constructs like matrices. For some of these more abstract constructs, the order in which the operands are multiplied together matters. A listing of the many different kinds of products used in mathematics is given in Product (mathematics).

The sign is encoded in Unicode at U+00D7 × MULTIPLICATION SIGN (HTML `×`

**·** `×`

).

In computer programming, the asterisk (as in `5*2`

) is still the most common notation. This is due to the fact that most computers historically were limited to small character sets (such as ASCII and EBCDIC) that lacked a multiplication sign (such as `⋅`

or `×`

), while the asterisk appeared on every keyboard. This usage originated in the FORTRAN programming language.

The numbers to be multiplied are generally called the "factors". The number to be multiplied is the "multiplicand", and the number by which it is multiplied is the "multiplier". Usually the multiplier is placed first and the multiplicand is placed second;^{[1]} however sometimes the first factor is the multiplicand and the second the multiplier.^{[5]} Also as the result of a multiplication does not depend on the order of the factors, the distinction between "multiplicand" and "multiplier" is useful only at a very elementary level and in some multiplication algorithms, such as the long multiplication. Therefore, in some sources, the term "multiplicand" is regarded as a synonym for "factor".^{[6]} In algebra, a number that is the multiplier of a variable or expression (e.g., the 3 in 3*xy*^{2}) is called a coefficient.

The result of a multiplication is called a product. A product of integers is a multiple of each factor. For example, 15 is the product of 3 and 5, and is both a multiple of 3 and a multiple of 5.

For example: set the monkey's feet to 4 and 9, and get the product – 36 – in its hands.The common methods for multiplying numbers using pencil and paper require a multiplication table of memorized or consulted products of small numbers (typically any two numbers from 0 to 9), however one method, the peasant multiplication algorithm, does not. The example below illustrates "long multiplication" (the "standard algorithm", "grade-school multiplication"):

23958233 × 5830 ——————————————— 00000000 ( = 23,958,233 × 0) 71874699 ( = 23,958,233 × 30) 191665864 ( = 23,958,233 × 800) + 119791165 ( = 23,958,233 × 5,000) ——————————————— 139676498390 ( = 139,676,498,390 )

Multiplying numbers to more than a couple of decimal places by hand is tedious and error prone. Common logarithms were invented to simplify such calculations, since adding logarithms is equivalent to multiplying. The slide rule allowed numbers to be quickly multiplied to about three places of accuracy. Beginning in the early 20th century, mechanical calculators, such as the Marchant, automated multiplication of up to 10 digit numbers. Modern electronic computers and calculators have greatly reduced the need for multiplication by hand.

Methods of multiplication were documented in the writings of ancient Egyptian, Greek, Indian and Chinese civilizations.

The Ishango bone, dated to about 18,000 to 20,000 BC, may hint at a knowledge of multiplication in the Upper Paleolithic era in Central Africa, but this is speculative.

The Egyptian method of multiplication of integers and fractions, documented in the Ahmes Papyrus, was by successive additions and doubling. For instance, to find the product of 13 and 21 one had to double 21 three times, obtaining 2 × 21 = 42, 4 × 21 = 2 × 42 = 84, 8 × 21 = 2 × 84 = 168. The full product could then be found by adding the appropriate terms found in the doubling sequence:

The Babylonians used a sexagesimal positional number system, analogous to the modern day decimal system. Thus, Babylonian multiplication was very similar to modern decimal multiplication. Because of the relative difficulty of remembering 60 × 60 different products, Babylonian mathematicians employed multiplication tables. These tables consisted of a list of the first twenty multiples of a certain *principal number* *n*: *n*, 2*n*, ..., 20*n*; followed by the multiples of 10*n*: 30*n* 40*n*, and 50*n*. Then to compute any sexagesimal product, say 53*n*, one only needed to add 50*n* and 3*n* computed from the table.

In the mathematical text *Zhoubi Suanjing*, dated prior to 300 BC, and the *Nine Chapters on the Mathematical Art*, multiplication calculations were written out in words, although the early Chinese mathematicians employed Rod calculus involving place value addition, subtraction, multiplication and division. The Chinese were already using a decimal multiplication table by the end of the Warring States period.^{[7]}

The modern method of multiplication based on the Hindu–Arabic numeral system was first described by Brahmagupta. Brahmagupta gave rules for addition, subtraction, multiplication and division. Henry Burchard Fine, then professor of Mathematics at Princeton University, wrote the following:

*The Indians are the inventors not only of the positional decimal system itself, but of most of the processes involved in elementary reckoning with the system. Addition and subtraction they performed quite as they are performed nowadays; multiplication they effected in many ways, ours among them, but division they did cumbrously.*

These place value decimal arithmetic algorithms were introduced to Arab countries by Al Khwarizmi in the early 9th century, and popularized in the Western world by Fibonacci in the 13th century.

Grid method multiplication or the box method, is used in primary schools in England and Wales and in some areas of the United States to help teach an understanding of how multiple digit multiplication works. An example of multiplying 34 by 13 would be to lay the numbers out in a grid like:

The classical method of multiplying two *n*-digit numbers requires *n*^{2} digit multiplications. Multiplication algorithms have been designed that reduce the computation time considerably when multiplying large numbers. Methods based on the discrete Fourier transform reduce the computational complexity to *O*(*n* log *n* log log *n*). Recently, the factor log log *n* has been replaced by a function that increases much slower although it is still not constant (as it can be hoped).^{[9]}

One can only meaningfully add or subtract quantities of the same type, but quantities of different types can be multiplied or divided without problem. For example, four bags with three marbles each can be thought of as:^{[1]}

When two measurements are multiplied together the product is of a type depending on the types of the measurements. The general theory is given by dimensional analysis. This analysis is routinely applied in physics, but has also applications found in finance and other applied fields.

A common example in physics is the fact that multiplying speed by time gives distance. For example:

In this case, the hour units cancel out, leaving the product with only kilometer units.

The subscript gives the symbol for a bound variable (*i* in this case), called the "index of multiplication", together with its lower bound (*1*), whereas the superscript (here *4*) gives its upper bound. The lower and upper bound are expressions denoting integers. The factors of the product are obtained by taking the expression following the product operator, with successive integer values substituted for the index of multiplication, starting from the lower bound and incremented by 1 up to (and including) the upper bound. For example:

where *m* and *n* are integers or expressions that evaluate to integers. In case where *m* = *n*, the value of the product is the same as that of the single factor *x*_{m}; if *m* > *n*, the product is an empty product whose value is 1—regardless of the expression for the factors.

If all factors are identical, a product of n factors is equivalent to an exponentiation:

One may also consider products of infinitely many terms; these are called infinite products. Notationally, this consists in replacing *n* above by the Infinity symbol ∞. The product of such an infinite sequence is defined as the limit of the product of the first *n* terms, as *n* grows without bound. That is,

For the real and complex numbers, which includes for example natural numbers, integers, and fractions, multiplication has certain properties:

Other mathematical systems that include a multiplication operation may not have all these properties. For example, multiplication is not, in general, commutative for matrices and quaternions.

In the book *Arithmetices principia, nova methodo exposita*, Giuseppe Peano proposed axioms for arithmetic based on his axioms for natural numbers.^{[16]} Peano arithmetic has two axioms for multiplication:

Here *S*(*y*) represents the successor of *y*, or the natural number that *follows* *y*. The various properties like associativity can be proved from these and the other axioms of Peano arithmetic including induction. For instance *S*(0), denoted by 1, is a multiplicative identity because

The axioms for integers typically define them as equivalence classes of ordered pairs of natural numbers. The model is based on treating (*x*,*y*) as equivalent to *x* − *y* when *x* and *y* are treated as integers. Thus both (0,1) and (1,2) are equivalent to −1. The multiplication axiom for integers defined this way is

Multiplication is extended in a similar way to rational numbers and then to real numbers.

The product of non-negative integers can be defined with set theory using cardinal numbers or the Peano axioms. See below how to extend this to multiplying arbitrary integers, and then arbitrary rational numbers. The product of real numbers is defined in terms of products of rational numbers, see construction of the real numbers.

There are many sets that, under the operation of multiplication, satisfy the axioms that define group structure. These axioms are closure, associativity, and the inclusion of an identity element and inverses.

A simple example is the set of non-zero rational numbers. Here we have identity 1, as opposed to groups under addition where the identity is typically 0. Note that with the rationals, we must exclude zero because under multiplication, it does not have an inverse: there is no rational number that can be multiplied by zero to result in 1. In this example we have an abelian group, but that is not always the case.

To see this, consider the set of invertible square matrices of a given dimension over a given field. Here, it is straightforward to verify closure, associativity, and inclusion of identity (the identity matrix) and inverses. However, matrix multiplication is not commutative, which shows that this group is non-abelian.

Another fact worth noticing is that the integers under multiplication is not a group—even if we exclude zero. This is easily seen by the nonexistence of an inverse for all elements other than 1 and −1.

Numbers can *count* (3 apples), *order* (the 3rd apple), or *measure* (3.5 feet high); as the history of mathematics has progressed from counting on our fingers to modelling quantum mechanics, multiplication has been generalized to more complicated and abstract types of numbers, and to things that are not numbers (such as matrices) or do not look much like numbers (such as quaternions).

When multiplication is repeated, the resulting operation is known as **exponentiation**. For instance, the product of three factors of two (2×2×2) is "two raised to the third power", and is denoted by 2^{3}, a two with a superscript three. In this example, the number two is the **base**, and three is the **exponent**. In general, the exponent (or superscript) indicates how many times the base appears in the expression, so that the expression

indicates that *n* copies of the base *a* are to be multiplied together. This notation can be used whenever multiplication is known to be power associative.