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| | Multiplication is also defined for other types of numbers, such as complex numbers, and for 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). | | Multiplication is also defined for other types of numbers, such as complex numbers, and for 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). |
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| − | ==Notation and terminology== | + | ==Properties== |
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| − | In arithmetic, multiplication is often written using the multiplication sign (either × or <math>\times</math>) between the terms (that is, in infix notation). For example,
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| − | :<math>2\times 3 = 6</math> ("two times three equals six")
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| − | :<math>3\times 4 = 12</math>
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| − | :<math>2\times 3\times 5 = 6\times 5 = 30</math>
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| − | :<math>2\times 2\times 2\times 2\times 2 = 32</math>
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| − | There are other mathematical notations for multiplication:
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| − | * To reduce confusion between the multiplication sign × and the common variable {{mvar|x}}, multiplication is also denoted by dot signs, usually a middle-position dot (rarely period):
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| − | :{{math|5 ⋅ 2}} or {{math|5 . 3}}
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| − | :The middle dot notation, encoded in Unicode as U+22C5 ⋅ DOT OPERATOR, is now standard in the United States and other countries where the period is used as a decimal point. When the dot operator character is not accessible, the interpunct (·) is used. In other countries that use a comma as a decimal mark, either the period or a middle dot is used for multiplication.
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| − | :Historically, in the United Kingdom and Ireland, the middle dot was sometimes used for the decimal to prevent it from disappearing in the ruled line, and the period/full stop was used for multiplication. However, since the Ministry of Technology ruled to use the period as the decimal point in 1968, and the SI standard has since been widely adopted, this usage is now found only in the more traditional journals such as ''The Lancet''.
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| − | * In algebra, multiplication involving variables is often written as a juxtaposition (e.g., ''xy'' for ''x'' times ''y'' or 5''x'' for five times ''x''), also called ''implied multiplication''. The notation can also be used for quantities that are surrounded by parentheses (e.g., 5(2) or (5)(2) for five times two). This implicit usage of multiplication can cause ambiguity when the concatenated variables happen to match the name of another variable, when a variable name in front of a parenthesis can be confused with a function name, or in the correct determination of the order of operations.
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| − | * In vector multiplication, there is a distinction between the cross and the dot symbols. The cross symbol generally denotes the taking a cross product of two vectors, yielding a vector as its result, while the dot denotes taking the dot product of two vectors, resulting in a scalar.
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| − | In computer programming, the asterisk (as in <code>5*2</code>) 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 <code>⋅</code> or <code>×</code>), while the asterisk appeared on every keyboard. This usage originated in the FORTRAN programming language.
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| − | 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; however sometimes the first factor is the multiplicand and the second the multiplier. Also, as the result of 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". In algebra, a number that is the multiplier of a variable or expression (e.g., the 3 in 3''xy''<sup>2</sup>) is called a coefficient.
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| − | The result of a multiplication is called a product. When one factor is an integer, the product is a multiple of the other or of the product of the others. Thus <math> 2 \times \pi </math> is a multiple of <math> \pi </math>, as is <math> 5133 \times 486 \times \pi </math>. 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.
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| − | ==Computation==
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| − | [[file:צעצוע מכני משנת 1918 לחישובי לוח הכפל The Educated Monkey.jpg|200px|right|thumb|The Educated Monkey – a tin toy dated 1918, used as a multiplication "calculator". <small>For example: set the monkey's feet to 4 and 9, and get the product – 36 – in its hands.</small>]]
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| − | Many 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"):
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| − | 23958233
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| − | × 5830
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| − | ———————————————
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| − | 00000000 ( = 23,958,233 × 0)
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| − | 71874699 ( = 23,958,233 × 30)
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| − | 191665864 ( = 23,958,233 × 800)
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| − | + 119791165 ( = 23,958,233 × 5,000)
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| − | ———————————————
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| − | 139676498390 ( = 139,676,498,390 )
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| − | 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.
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| − | ===Historical algorithms===
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| − | Methods of multiplication were documented in the writings of ancient Egyptian, Greek, Indian, and Chinese civilizations.
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| − | 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.
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| − | ====Egyptians====
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| − | The Egyptian method of multiplication of integers and fractions, which is documented in the Rhind Mathematical 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:
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| − | :13 × 21 = (1 + 4 + 8) × 21 = (1 × 21) + (4 × 21) + (8 × 21) = 21 + 84 + 168 = 273.
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| − | ====Babylonians====
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| − | 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.
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| − | ====Chinese====
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| − | [[File:Multiplication algorithm.GIF|thumb|right|250px|38 × 76 = 2888]]
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| − | 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.
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| − | ===Modern methods===
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| − | [[Image:Gelosia multiplication 45 256.png|right|250px|thumb|Product of 45 and 256. Note the order of the numerals in 45 is reversed down the left column. The carry step of the multiplication can be performed at the final stage of the calculation (in bold), returning the final product of 45 × 256 = 11520. This is a variant of Lattice multiplication.]]
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| − | 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 a professor of mathematics at Princeton University, wrote the following:
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| − | :''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.''
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| − | 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.
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| − | ====Grid method====
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| − | 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 as follows:
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| − | :{| class="wikitable" style="text-align: center;"
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| − | ! scope="col" width="40pt" |
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| − | ! scope="col" width="120pt" | 30
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| − | ! scope="col" width="40pt" | 4
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| − | |-
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| − | ! scope="row" | 10
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| − | |300
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| − | |40
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| − | |-
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| − | ! scope="row" | 3
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| − | |90
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| − | |12
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| − | |}
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| − |
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| − | and then add the entries.
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| − | ===Computer algorithms===
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| − | The classical method of multiplying two {{math|''n''}}-digit numbers requires {{math|''n''<sup>2</sup>}} 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 {{math|''O''(''n'' log ''n'' log log ''n'')}}. In 2016, the factor {{math|log log ''n''}} was replaced by a function that increases much slower, though still not constant. In March 2019, David Harvey and Joris van der Hoeven submitted a paper presenting an integer multiplication algorithm with a complexity of <math>O(n\log n).</math> The algorithm, also based on the fast Fourier transform, is conjectured to be asymptotically optimal. The algorithm is not practically useful, as it only becomes faster for multiplying extremely large numbers (having more than {{math|2<sup>1729<sup>12</sup></sup>}} bits).
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| − | ==Products of measurements==
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| − | One can only meaningfully add or subtract quantities of the same type, but quantities of different types can be multiplied or divided without problems. For example, four bags with three marbles each can be thought of as:
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| − | :[4 bags] × [3 marbles per bag] = 12 marbles.
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| − | When two measurements are multiplied together, the product is of a type depending on the types of measurements. The general theory is given by dimensional analysis. This analysis is routinely applied in physics, but it also has applications in finance and other applied fields.
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| − | A common example in physics is the fact that multiplying speed by time gives distance. For example:
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| − | :50 kilometers per hour × 3 hours = 150 kilometers.
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| − | In this case, the hour units cancel out, leaving the product with only kilometer units.
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| − | Other examples of multiplication involving units include:
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| − | :2.5 meters × 4.5 meters = 11.25 square meters
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| − | :11 meters/seconds × 9 seconds = 99 meters
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| − | :4.5 residents per house × 20 houses = 90 residents
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| − | ==Product of a sequence==
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| − | ===Capital pi notation===
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| − | The product of a sequence of factors can be written with the product symbol, which derives from the capital letter <math>\textstyle \prod</math> (pi) in the Greek alphabet (much like the same way the capital letter <math>\textstyle \sum</math> (sigma) is used in the context of summation). Unicode position U+220F ∏ contains a glyph for denoting such a product, distinct from U+03A0 Π, the letter. The meaning of this notation is given by:
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| − | :<math>\prod_{i=1}^4 i = 1\cdot 2\cdot 3\cdot 4,</math>
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| − | that is
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| − | :<math>\prod_{i=1}^4 i = 24.</math>
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| − | 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:
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| − | :<math>\prod_{i=1}^6 i = 1\cdot 2\cdot 3\cdot 4\cdot 5 \cdot 6 = 720.</math>
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| − |
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| − | More generally, the notation is defined as
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| − | :<math>\prod_{i=m}^n x_i = x_m \cdot x_{m+1} \cdot x_{m+2} \cdot \,\,\cdots\,\, \cdot x_{n-1} \cdot x_n,</math>
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| − | where ''m'' and ''n'' are integers or expressions that evaluate to integers. In the case where ''m'' = ''n'', the value of the product is the same as that of the single factor ''x''<sub>''m''</sub>; if ''m'' > ''n'', the product is an empty product whose value is 1—regardless of the expression for the factors.
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| − | ==== Properties of capital pi notation====
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| − | By definition,
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| − | :<math>\prod_{i=1}^{n}x_i=x_1\cdot x_2\cdot\ldots\cdot x_n.</math>
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| − | If all factors are identical, a product of {{mvar|n}} factors is equivalent to exponentiation:
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| − | :<math>\prod_{i=1}^{n}x=x\cdot x\cdot\ldots\cdot x=x^n.</math>
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| − |
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| − | Associativity and commutativity of multiplication imply
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| − | :<math>\prod_{i=1}^{n}{x_iy_i} =\left(\prod_{i=1}^{n}x_i\right)\left(\prod_{i=1}^{n}y_i\right)</math> and
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| − | :<math>\left(\prod_{i=1}^{n}x_i\right)^a =\prod_{i=1}^{n}x_i^a</math>
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| − | if {{mvar|a}} is a nonnegative integer, or if all <math>x_i</math> are positive real numbers, and
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| − | :<math>\prod_{i=1}^{n}x^{a_i} =x^{\sum_{i=1}^{n}a_i}</math>
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| − | if all <math>a_i</math> are nonnegative integers, or if {{mvar|x}} is a positive real number.
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| − |
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| − | ===Infinite products===
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| − | 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,
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| − | :<math>\prod_{i=m}^\infty x_i = \lim_{n\to\infty} \prod_{i=m}^n x_i.</math>
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| − | One can similarly replace ''m'' with negative infinity, and define:
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| − | :<math>\prod_{i=-\infty}^\infty x_i = \left(\lim_{m\to-\infty}\prod_{i=m}^0 x_i\right) \cdot \left(\lim_{n\to\infty} \prod_{i=1}^n x_i\right),</math>
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| − | provided both limits exist.
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| − | ==Properties==
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| − | [[Image:Multiplication chart.svg|thumb|right|Multiplication of numbers 0–10. Line labels = multiplicand. X-axis = multiplier. Y-axis = product.<br>Extension of this pattern into other quadrants gives the reason why a negative number times a negative number yields a positive number.<br>Note also how multiplication by zero causes a reduction in dimensionality, as does multiplication by a singular matrix where the determinant is 0. In this process, information is lost and cannot be regained.]]
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| | For real and complex numbers, which includes, for example, natural numbers, integers, and fractions, multiplication has certain properties: | | For real and complex numbers, which includes, for example, natural numbers, integers, and fractions, multiplication has certain properties: |
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| | 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. | | 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. |
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| − | ==Axioms==
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| − | In the book ''Arithmetices principia, nova methodo exposita'', Giuseppe Peano proposed axioms for arithmetic based on his axioms for natural numbers. Peano arithmetic has two axioms for multiplication:
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| − | :<math>x \times 0 = 0</math>
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| − | :<math>x \times S(y) = (x \times y) + x</math>
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| − | Here ''S''(''y'') represents the successor of ''y''; i.e., 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
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| − | :<math>x \times 1 = x \times S(0) = (x \times 0) + x = 0 + x = x.</math>
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| − |
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| − | 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
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| − | :<math>(x_p,\, x_m) \times (y_p,\, y_m) = (x_p \times y_p + x_m \times y_m,\; x_p \times y_m + x_m \times y_p).</math>
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| − |
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| − | The rule that −1 × −1 = 1 can then be deduced from
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| − | :<math>(0, 1) \times (0, 1) = (0 \times 0 + 1 \times 1,\, 0 \times 1 + 1 \times 0) = (1,0).</math>
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| − |
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| − | Multiplication is extended in a similar way to rational numbers and then to real numbers.
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| − | ==Multiplication with set theory==
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| − | 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.
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| | == Licensing == | | == Licensing == |
| | Content obtained and/or adapted from: | | Content obtained and/or adapted from: |
| | * [https://en.wikipedia.org/wiki/Multiplication Multiplication, Wikipedia] under a CC BY-SA license | | * [https://en.wikipedia.org/wiki/Multiplication Multiplication, Wikipedia] under a CC BY-SA license |
Four bags with three marbles per bag gives twelve marbles (4 × 3 = 12).
Multiplication can also be thought of as scaling. Here we see 2 being multiplied by 3 using scaling, giving 6 as a result.
Animation for the multiplication 2 × 3 = 6.
4 × 5 = 20. The large rectangle is made up of 20 squares, each 1 unit by 1 unit.
Area of a cloth 4.5m × 2.5m = 11.25m
2;
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 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.
For example, 4 multiplied by 3, often written as 
and spoken as "3 times 4", can be calculated by adding 3 copies of 4 together:
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.
Systematic generalizations of this basic definition define the multiplication of integers (including negative numbers), rational numbers (fractions), and real numbers.
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 a product 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 for 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).
Properties
For real and complex numbers, which includes, for example, natural numbers, integers, and fractions, multiplication has certain properties:
- Commutative property
- The order in which two numbers are multiplied does not matter:
- Associative property
- Expressions solely involving multiplication or addition are invariant with respect to the order of operations:
- Distributive property
- Holds with respect to multiplication over addition. This identity is of prime importance in simplifying algebraic expressions:
- Identity element
- The multiplicative identity is 1; anything multiplied by 1 is itself. This feature of 1 is known as the identity property:
- Property of 0
- Any number multiplied by 0 is 0. This is known as the zero property of multiplication:
- Negation
- −1 times any number is equal to the additive inverse of that number.
where 
- –1 times –1 is 1.
- Inverse element
- Every number x, except 0, has a multiplicative inverse,
, such that 
.
- Order preservation
- Multiplication by a positive number preserves the order:
- For a > 0, if b > c then ab > ac.
- Multiplication by a negative number reverses the order:
- For a < 0, if b > c then ab < ac.
- The complex numbers do not have an ordering that is compatible with both addition and multiplication.
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.
Licensing
Content obtained and/or adapted from:
