## What is the closure of the empty set?

In any discrete space, since every set is closed (and also open), every set is equal to its closure.

In any indiscrete space X, since the only closed sets are the empty set and X itself, we have that the closure of the empty set is the empty set, and for every non-empty subset A of X, cl(A) = X..

## Is QA closed set?

Q is not closed because it is dense, and if a set is both dense and closed then it is equal to the whole space (in this case, R). Q is not open either because open sets are either empty, or contain an interval which makes them uncountable; but Q is countably infinite so it is neither empty nor uncountable.

## Is 0 to infinity closed?

This set is indeed closed. Note that +∞ is not a real number, sequences which tend to it are therefore non-convergent and have no limit in R. From this we can easily infer that [0,∞) is closed, since every sequence of positive numbers converging to a limit would have a non-negative limit which is in [0,∞).

## Is Q closed or open in R?

6 Answers. In the usual topology of R, Q is neither open nor closed. The interior of Q is empty (any nonempty interval contains irrationals, so no nonempty open set can be contained in Q). Since Q does not equal its interior, Q is not open.

## Is a closed set?

In geometry, topology, and related branches of mathematics, a closed set is a set whose complement is an open set. In a topological space, a closed set can be defined as a set which contains all its limit points. In a complete metric space, a closed set is a set which is closed under the limit operation.

## Is an empty set bounded?

The set of all real numbers is the only interval that is unbounded at both ends; the empty set (the set containing no elements) is bounded. An interval that has only one real-number endpoint is said to be half-bounded, or more descriptively, left-bounded or right-bounded.

## Can an infinite set be closed?

Similarly, every finite or infinite closed interval [a, b], (−∞,b], or [a, ∞) is closed. The empty set ∅ and R are both open and closed; they’re the only such sets. Most subsets of R are neither open nor closed (so, unlike doors, “not open” doesn’t mean “closed” and “not closed” doesn’t mean “open”).

## How do you know if a set is closed?

One way to determine if you have a closed set is to actually find the open set. The closed set then includes all the numbers that are not included in the open set. For example, for the open set x < 3, the closed set is x >= 3. This closed set includes the limit or boundary of 3.

## Is Infinity bounded?

In theory, you can go on counting forever without ever reaching a largest number. However, infinity can be bounded, too, like the infinity symbol, for example. You can loop around it an unlimited number of times, but you must follow its contour—or boundary.

## Is 0 open or closed?

{0} is not open because it does not contain any neighborhood of the point x = 1. For the last question, we need to look at the complement of the set {1, 1/2, 1/3, 1/4, 1/5, … }

## Why is R both open and closed?

Originally Answered: Is R (real number) is closed or open? Both. The set of real numbers is open because every point in the set has an open neighbourhood of other points also in the set. A rough intuition is that it is open because every point is in the interior of the set.