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Is there a subset of the natural numbers that is closed for addition?
Yes. The entire set of natural numbers is closed under addition (but not subtraction). So are the even numbers (but not the odd numbers), the multiples of 3, of 4, etc.
What is a set of rational numbers not closed under?
It is not closed under taking square (or other even) roots.
Are even natural numbers closed under division?
Yesss.
Is the set of integers is close under subtraction?
Yes, because suppose that 'a' and 'b' are both arbitrary integers. Then (a-b) or (b-a) will then provide you with another integer. Suppose that the integer you are given from (a-b) is not unique. Then we have: (a-b)=c and (a-b)=c' Then, trivially, since (a-b)=(a-b), we have c=c'. Thus it is closed under subtraction.
Why are even whole and natural numbers closed under addition?
For a set to be closed under an operation, doing the operations on any two members of the set must result in another member of the set.The set of even natural numbers is {2, 4, 6, ...}.When two even numbers are added, the result is also an even number.Adding two positive numbers results in a larger positive number.Thus, adding two even natural numbers together results in another even natural number and so is closed.The set of even whole numbers is {..., -6, -4, -2, 0, 2, 4, 6, ...}.When two even numbers are added, the result is also an even number.It does not matter which combination of positive and negative whole numbers are added as all positive and negative whole numbers (including zero) are in the set. Thus it is also closed under addition.
