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Division by zero
In mathematics, a division is called a division by zero if the divisor is zero. Such a division can be formally expressed as , where a is the dividend. Whether or not this expression can be assigned a meaningful (well-defined) value depends upon how the expression is interpreted.
It is generally regarded among mathematicians that a natural way to interpret division by zero is to first define division in terms of other arithmetic operations. Under the standard rules for arithmetic on integers, rational numbers, real numbers and complex numbers, the value of a division by zero is undefined. The reason is that division is defined to be the inverse operation of multiplication. This means that the value of
is the solution x of the equation
whenever such a value exists and is unique. Otherwise the expression is undefined.
For b = 0, the equation bx = a can be rewritten as 0x = a or simply 0 = a. Thus, in this case, the equation bx = a has no solution if a is not equal to 0, and has any x as a solution if a equals 0. In either case, is undefined. Conversely, for the number systems mentioned above, the expression is always defined if b is not equal to zero.
Fallacies based on division by zero
- For any real number x:
- Factoring both sides in two different ways:
- Dividing both sides by x − x:
- Since this is valid for any value of x, we can plug in x = 1.
The fallacy is in the assumption that division by x − x = 0 is defined.
In practice, division by a term in any algebraic argument will require either an explicit assumption that the term is not zero, or a separate justification showing that the term can never be zero.
Similar statements are true in more general algebraic structures, such as rings and fields. In a field, every nonzero element is invertible under multiplication, so as above, division only poses problems when attempting to divide by zero. However, in other rings, division by nonzero elements may also pose problems. Consider, for example, the ring Z/6Z of integers mod 6. What meaning should we give to the expression
This should be the solution x of the equation
But this equation has two distinct solutions, x = 1 and x = 4, so the expression is undefined. The problem occurs because 2 is not invertible under multiplication.
Limits and division by zero
At first glance it seems possible to define by considering the limit of as b approaches 0. For any nonzero a, it is known that
Therefore, we might consider defining as for positive a, and for negative a. However, this definition is not generally useful, because positive and negative infinity are not real numbers, and the equation
still has no solution for any finite a. Furthermore, there is no obvious definition of that can be derived from considering the limit of a ratio. The limit
does not exist. Limits of the form
in which both f(x) and g(x) approach 0 as x approaches 0, may converge to any value or may not converge at all. See l'Hopital's rule for discussion and examples of limits of ratios.
In distribution theory one can extend the function
to a distribution on the whole space of real numbers (in effect by using Cauchy principal values). It does not, however, make sense to ask for a 'value' of this distribution at x = 0; a sophisticated answer refers to the singular support of the distribution.
Other number systems
Although division by zero is undefined with real numbers and integers, it is possible to consistently define division by zero in other mathematical structures, for instance on the Riemann sphere (see also poles in complex analysis). In hyperreal numbers and surreal numbers, division by non-zero infinitesimals is possible. If a number system forms a commutative ring, as do the integers, the real numbers, and the complex numbers, for instance, it can be extended to a wheel in which division by zero is always possible, but division has then a slightly different meaning.
Division by zero in computer arithmetic
IEEE 754 specifies that every floating point arithmetic operation, including division by zero, has a well-defined result. In IEEE 754 arithmetic, a/0 is positive infinity when a is positive, negative infinity when a is negative, and NaN ("not a number") when a = 0. These definitions are derived from the properties of limits of ratios, as discussed above. At present, IEEE 754 is the most common floating point specification, as it is implemented by Intel processors and others.
Integer division by zero may be handled differently than floating point. Intel processors generate an interrupt when an attempt is made to divide an integer by zero. The usual result is aborting whatever program it occurred in.
To ensure that every operation yields a finite, numerical result (floating point), and avoids an interrupt (integer), a computer program may refuse to execute a division if the divisor is zero.
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