# Realsets

`realsets.py`

is a Sage module to support subsets of the real field consisting of intervals and isolated points and was developed to demonstrate set operations of the MGL `Set1`

module.

It is based of previous work from Interval1Sage adding integration on real sets and real intervals.

An object in this module consists of a list of disjoint open intervals plus a list of isolated points (not belonging to these intervals). Notice that `Infinite`

is acceptable as interval bound. Therefore, one can define:

- All sort of real intervals: open, close and half-open
- Finite sets
- Unbounded intervals
- And combinations of these by union, intersection and taking complements.

Represent a set that can be the union of some intervals and isolated points. It consists of:

- A list of disjoint open non-empty intervals.
- A list of points. Each of these points belongs at most to one interval.

## Examples

A closed interval:

```
? RealSet.cc_interval(1,4);
[ 1 :: 4 ]
```

A single point:

```
? RealSet.singleton(1)
{1}
```

#### Union

Union is supported with intervals and can be nested :

```
? I = RealSet.co_interval(1, 4)
? J = RealSet.co_interval(4, 5)
? M = RealSet.oc_interval(7, 8)
? I.union(J).union(M)
[ 1 :: 5 [ ∪ ] 7 :: 8 ]
```

#### Intersection

```
? I.intersection(J)
()
? I.intersection(RealSet.cc_interval(2,5))
[ 2 :: 4 [
```

#### Queries

Is a point in the set?

```
? I = RealSet.oo_interval(1, 3)
? 2 in I
True
? 3 in I
False
```

Is a set discrete (i.e: does not contain intervals)?

```
? RealSet.oo_interval(0,1).discrete
False
? RealSet(points=(1,2,3)).discrete
True
```

Size of a discrete is the number of points:

```
? RealSet(points=range(5)).size
5
? RealSet.oo_interval(0,3).size
+Infinity
```

A is subset of B

```
? A = RealSet.oo_interval(0,1)
? B = RealSet.cc_interval(0,1)
? RealSet().subset(A)
True
? B.subset(A)
False
? A.subset(B)
True
? A.subset(A)
True
? A.subset(A, proper=True)
False
```

Return the infimum (greatest lower bound)

```
? RealSet(points=range(3)).infimum()
0
? RealSet.oo_interval(1,3).infimum()
1
```

The opposite of a set: –A = {-x | x ∈ A}

```
? -RealSet.oo_interval(1,2)
] -2 :: -1 [
```

Return the supremum (least upper bound)

```
? RealSet(points=range(3)).supremum()
2
? RealSet.oo_interval(1,3).supremum()
3
```

The complementary of a set:

```
? RealSet.oo_interval(2,3).complement()
] -Infinity :: 2 ] ∪ [ 3 :: +Infinity [
? RealSet(points=range(3)).complement()
] 0 :: 1 [ ∪ ] 1 :: 2 [ ∪ ] 2 :: +Infinity [ ∪ ] -Infinity :: 0 [
```

The set difference of `A`

and `B`

: `\{x \in A, x\notin B\}`

```
? I = RealSet.oo_interval(2,+Infinity)
? J = RealSet.oo_interval(-Infinity, 5)
? I.setdiff(J)
[ 5 :: +Infinity [
? J.setdiff(I)
] -Infinity :: 2 ]
```

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