closure property define
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Closure Property
Closure Property
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Updated Jul 20 2020
Closure Property
The closure property means that a set is closed for some mathematical operation. That is, a set is closed with respect to that operation if the operation can always be completed with elements in the set. Thus, a set either has or lacks closure with respect to a given operation.
For example, the set of even natural numbers, [2, 4, 6, 8, . . .], is closed with respect to addition because the sum of any two of them is another even natural number, which is also a member of the set. (Natural numbers are defined as the set: [1, 2, 3, 4, . . .].) It is not closed with respect to division because the quotients 6/2 and 4/8, for example, cannot be computed without using odd numbers (6/2 = 3) or fractions (4/8 = ½g;), which are not members of the set.
Knowing the operations for which a given set is closed helps one understand the nature of the set. Thus, one knows that the set of natural numbers is less versatile than the set of integers because the latter is closed with respect to subtraction, but the former is not. (Integers are defined as the set: [. . .-3, -2, -1, 0, 1, 2, 3, . . .].) Similarly one knows that the set of polynomials is much like the set of integers because both sets are closed under addition, multiplication, negation, and subtraction, but are not closed under division.
Particularly interesting examples of closure are the positive and negative numbers. In mathematical structure, these two sets are indistinguishable except for one property, closure with respect to multiplication. Once one decides that the product of two positive numbers is positive, the other rules for multiplying and dividing various combinations of positive and negative numbers follow. Then, for example, the product of two negative numbers must be positive, and so on.
The lack of closure is one reason for enlarging a set. For example, without augmenting the set of rational numbers with the irrationals, one cannot solve an equation such as x2 = 2, which can arise from the use of the pythagorean theorem. Without extending the set of real numbers to include imaginary numbers, one cannot solve an equation such as x2 + 1 = 0, contrary to the fundamental theorem of algebra.