Negative number

thumb|Thermometer indicating a negative [[Fahrenheit temperature (−4 °F).]]

In mathematics, a negative number is the opposite of a positive real number. Equivalently, a negative number is a real number that is less than zero. Negative numbers are often used to represent the magnitude of a loss or deficiency. A debt that is owed may be thought of as a negative asset. If a quantity, such as the charge on an electron, may have either of two opposite senses, then one may choose to distinguish between those senses—perhaps arbitrarily—as positive and negative. Negative numbers are used to describe values on a scale that goes below zero, such as the Celsius and Fahrenheit scales for temperature. The laws of arithmetic for negative numbers ensure that the common-sense idea of an opposite is reflected in arithmetic. For example, −(−3) = 3 because the opposite of an opposite is the original value.

Negative numbers are usually written with a minus sign in front. For example, −3 represents a negative quantity with a magnitude of three, and is pronounced and read as "minus three" or "negative three". Conversely, a number that is greater than zero is called positive; zero is usually (but not always) thought of as neither positive nor negative. The positivity of a number may be emphasized by placing a plus sign before it, e.g. +3. In general, the negativity or positivity of a number is referred to as its sign.

Every real number other than zero is either positive or negative. The non-negative whole numbers are referred to as natural numbers (i.e., 0, 1, 2, 3, ...), while the positive and negative whole numbers (together with zero) are referred to as integers. (Some definitions of the natural numbers exclude zero.)

In bookkeeping, amounts owed are often represented by red numbers, or a number in parentheses, as an alternative notation to represent negative numbers.

Negative numbers were used in the Nine Chapters on the Mathematical Art, which in its present form dates from the period of the Chinese Han dynasty (202 BC – AD 220), but may well contain much older material. Liu Hui (c. 3rd century) established rules for adding and subtracting negative numbers. Western mathematicians like Leibniz held that negative numbers were invalid, but still used them in calculations.

Introduction

The number line

The relationship between negative numbers, positive numbers, and zero is often expressed in the form of a number line:

center|The number line

Numbers appearing farther to the right on this line are greater, while numbers appearing farther to the left are lesser. Thus zero appears in the middle, with the positive numbers to the right and the negative numbers to the left.

Note that a negative number with greater magnitude is considered less. For example, even though (positive) is greater than (positive) , written

negative is considered to be less than negative :

Signed numbers

In the context of negative numbers, a number that is greater than zero is referred to as positive. Thus every real number other than zero is either positive or negative, while zero itself is not considered to have a sign. Positive numbers are sometimes written with a plus sign in front, e.g. denotes a positive three.

Because zero is neither positive nor negative, the term nonnegative is sometimes used to refer to a number that is either positive or zero, while nonpositive is used to refer to a number that is either negative or zero. Zero is a neutral number.

As the result of subtraction

Negative numbers can be thought of as resulting from the subtraction of a larger number from a smaller. For example, negative three is the result of subtracting three from zero:

In general, the subtraction of a larger number from a smaller yields a negative result, with the magnitude of the result being the difference between the two numbers. For example,

since .

Everyday uses of negative numbers

Sport

Goal difference in association football and hockey; points difference in rugby football; net run rate in cricket; golf scores relative to par.
Plus-minus differential in ice hockey: the difference in total goals scored for the team (+) and against the team (−) when a particular player is on the ice is the player's +/− rating. Players can have a negative (+/−) rating.
Run differential in baseball: the run differential is negative if the team allows more runs than they scored.
Clubs may be deducted points for breaches of the laws, and thus have a negative points total until they have earned at least that many points that season.
Lap (or sector) times in Formula 1 may be given as the difference compared to a previous lap (or sector) (such as the previous record, or the lap just completed by a driver in front), and will be positive if slower and negative if faster.
In some athletics events, such as sprint races, the hurdles, the triple jump and the long jump, the wind assistance is measured and recorded, and is positive for a tailwind and negative for a headwind.

Science

Temperatures which are colder than 0 °C or 0 °F.
Latitudes south of the equator and longitudes west of the prime meridian.
Topographical features of the earth's surface are given a height above sea level, which can be negative (e.g. the surface elevation of the Dead Sea or Death Valley, or the elevation of the Thames Tideway Tunnel).
Electrical circuits. When a battery is connected in reverse polarity, the voltage applied is said to be the opposite of its rated voltage. For example, a 6-volt battery connected in reverse applies a voltage of −6 volts.
Ions have a positive or negative electrical charge.

Finance

Financial statements can include negative balances, indicated either by a minus sign or by enclosing the balance in parentheses. Examples include bank account overdrafts and business losses (negative earnings).
The annual percentage growth in a country's GDP might be negative, which is one indicator of being in a recession.
Occasionally, a rate of inflation may be negative (deflation), indicating a fall in average prices.
The daily change in a share price or stock market index, such as the FTSE 100 or the Dow Jones.
A negative number in financing is synonymous with "debt" and "deficit" which are also known as "being in the red".
Interest rates can be negative, when the lender is charged to deposit their money.

Other

thumb|right|Negative story numbers in an elevator.

The numbering of stories in a building below the ground floor.
When playing an audio file on a portable media player, such as an iPod, the screen display may show the time remaining as a negative number, which increases up to zero time remaining at the same rate as the time already played increases from zero.
Television game shows:
Participants on QI often finish with a negative points score.
Teams on University Challenge have a negative score if their first answers are incorrect and interrupt the question.
Jeopardy! has a negative money score – contestants play for an amount of money and any incorrect answer that costs them more than what they have now can result in a negative score.
In The Price Is Rights pricing game Buy or Sell, if an amount of money is lost that is more than the amount currently in the bank, it incurs a negative score.
The change in support for a political party between elections, known as swing.
A politician's approval rating.
In video games, a negative number indicates loss of life, damage, a score penalty, or consumption of a resource, depending on the genre of the simulation.
Employees with flexible working hours may have a negative balance on their timesheet if they have worked fewer total hours than contracted to that point. Employees may be able to take more than their annual holiday allowance in a year, and carry forward a negative balance to the next year.
Transposing notes on an electronic keyboard are shown on the display with positive numbers for increases and negative numbers for decreases, e.g. "−1" for one semitone down.

Arithmetic involving negative numbers

The minus sign "−" signifies the operator for both the binary (two-operand) operation of subtraction (as in ) and the unary (one-operand) operation of negation (as in , or twice in ). A special case of unary negation occurs when it operates on a positive number, in which case the result is a negative number (as in ).

The ambiguity of the "−" symbol does not generally lead to ambiguity in arithmetical expressions, because the order of operations makes only one interpretation or the other possible for each "−". However, it can lead to confusion and be difficult for a person to understand an expression when operator symbols appear adjacent to one another. A solution can be to parenthesize the unary "−" along with its operand.

For example, the expression may be clearer if written (even though they mean exactly the same thing formally). The subtraction expression is a different expression that doesn't represent the same operations, but it evaluates to the same result.

Sometimes in elementary schools a number may be prefixed by a superscript minus sign or plus sign to explicitly distinguish negative and positive numbers as in

Addition

right|thumb|A visual representation of the addition of positive and negative numbers. Larger balls represent numbers with greater magnitude.

Addition of two negative numbers is very similar to addition of two positive numbers. For example,

The idea is that two debts can be combined into a single debt of greater magnitude.

When adding together a mixture of positive and negative numbers, one can think of the negative numbers as positive quantities being subtracted. For example:

In the first example, a credit of is combined with a debt of , which yields a total credit of . If the negative number has greater magnitude, then the result is negative:

Here the credit is less than the debt, so the net result is a debt.

Subtraction

As discussed above, it is possible for the subtraction of two non-negative numbers to yield a negative answer:

In general, subtraction of a positive number yields the same result as the addition of a negative number of equal magnitude. Thus

and

On the other hand, subtracting a negative number yields the same result as the addition a positive number of equal magnitude. (The idea is that losing a debt is the same thing as gaining a credit.) Thus

and

Multiplication

thumb|A multiplication by a negative number can be seen as a change of direction of the [[Vector (mathematics and physics)|vector of magnitude equal to the absolute value of the product of the factors.]]

When multiplying numbers, the magnitude of the product is always just the product of the two magnitudes. The sign of the product is determined by the following rules:

The product of one positive number and one negative number is negative.
The product of two negative numbers is positive.

Thus

and

The reason behind the first example is simple: adding three s together yields :

The reasoning behind the second example is more complicated. The idea again is that losing a debt is the same thing as gaining a credit. In this case, losing two debts of three each is the same as gaining a credit of six:

The convention that a product of two negative numbers is positive is also necessary for multiplication to follow the distributive law. In this case, we know that

Since , the product must equal .

These rules lead to another (equivalent) rule—the sign of any product a × b depends on the sign of a as follows:

if a is positive, then the sign of a × b is the same as the sign of b, and
if a is negative, then the sign of a × b is the opposite of the sign of b.

The justification for why the product of two negative numbers is a positive number can be observed in the analysis of complex numbers.

Division

The sign rules for division are the same as for multiplication. For example,

and

If dividend and divisor have the same sign, the result is positive, if they have different signs the result is negative.

Negation

The negative version of a positive number is referred to as its negation. For example, is the negation of the positive number . The sum of a number and its negation is equal to zero:

That is, the negation of a positive number is the additive inverse of the number.

Using algebra, we may write this principle as an algebraic identity:

This identity holds for any positive number . It can be made to hold for all real numbers by extending the definition of negation to include zero and negative numbers. Specifically:

The negation of 0 is 0, and
The negation of a negative number is the corresponding positive number.

For example, the negation of is . In general,

The absolute value of a number is the non-negative number with the same magnitude. For example, the absolute value of and the absolute value of are both equal to , and the absolute value of is .

Formal construction of negative integers

In a similar manner to rational numbers, we can extend the natural numbers <math>\mathbb{N}</math> to the integers <math>\mathbb{Z}</math> by defining integers as an ordered pair of natural numbers (a, b). We can extend addition and multiplication to these pairs with the following rules:

We define an equivalence relation ~ upon these pairs with the following rule:

This equivalence relation is compatible with the addition and multiplication defined above, and we may define <math>\mathbb{Z}</math> to be the quotient set <math>\mathbb{N}^2/\sim</math>, i.e. we identify two pairs (a, b) and (c, d) if they are equivalent in the above sense. Note that <math>\mathbb{Z}</math>, equipped with these operations of addition and multiplication, is a ring, and is in fact, the prototypical example of a ring.

We can also define a total order on <math>\mathbb{Z}</math> by writing

This will lead to an additive zero of the form (a, a), an additive inverse of (a, b) of the form (b, a), a multiplicative unit of the form (a + 1, a), and a definition of subtraction

This construction is a special case of the Grothendieck construction.

Uniqueness

The additive inverse of a number is unique, as is shown by the following proof. As mentioned above, an additive inverse of a number is defined as a value which when added to the number yields zero.

Let x be a number and let y be its additive inverse. Suppose y′ is another additive inverse of x. By definition,

And so, x + y′ = x + y. Using the law of cancellation for addition, it is seen that y′ = y. Thus y is equal to any other additive inverse of x. That is, y is the unique additive inverse of x.

History

For a long time, understanding of negative numbers was delayed by the impossibility of having a negative-number amount of a physical object, for example "minus-three apples", and negative solutions to problems were considered "false".

In Hellenistic Egypt, the Greek mathematician Diophantus in the 3rd century AD referred to an equation that was equivalent to <math>4x + 20 = 4</math> (which has a negative solution) in Arithmetica, saying that the equation was absurd. For this reason Greek geometers were able to solve geometrically all forms of the quadratic equation which give positive roots, while they could take no account of others.

Negative numbers appear for the first time in history in the Nine Chapters on the Mathematical Art (九章算術, Jiǔ zhāng suàn-shù), which in its present form dates from the Han period, but may well contain much older material. The Chinese were able to solve simultaneous equations involving negative numbers. The Nine Chapters used red counting rods to denote positive coefficients and black rods for negative. This system is the exact opposite of contemporary printing of positive and negative numbers in the fields of banking, accounting, and commerce, wherein red numbers denote negative values and black numbers signify positive values. Liu Hui writes: