Documentation

intersect, _intersect

Intersection of sets or intervals or both

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Syntax

set1 intersect set2
_intersect(set1, set2, …)

Description

intersect computes the intersection of sets and intervals.

set1 intersect set2 is equivalent to _intersect(set1, set2).

The precedences of intersect, minus, union are as follows. If in doubt, use parentheses to ensure that the expression is parsed as desired.

• The operator intersect is stronger binding than minus, that is, set1 intersect set2 minus set3 = (set 1 intersect set2) minus set3.

• The operator minus is stronger binding than union, that is, set1 minus set2 union set3 = (set1 minus set2) union set3.

• set1 minus set2 minus set3 = (set 1 minus set2) minus set3

If sets or intervals are specified by symbolic expressions involving identifiers or indexed identifiers, then symbolic calls of _intersect are returned. On the screen, they are represented via the operator notation set1 intersect set2.

Note

On finite sets of type DOM_SET, intersect acts in a purely syntactical way. For example, {1} intersect {x} simplifies to an empty set {}. Mathematically, this result can be incorrect in general, because x can represent the value 1.

On intervals of type Dom::Interval, intersect acts in a semantical way. In particular, properties of identifiers are taken into account.

_intersect() returns universe (of type stdlib::Universe), which represents the set of all mathematical objects.

Examples

Example 1

intersect operates on finite sets:

{x, 1, 5} intersect {x, 1, 3, 4} For symbolic sets, specified as identifiers or indexed identifiers, symbolic calls are returned:

{1, 2} intersect A intersect {2, 3} Note that intersect acts on finite sets in a purely syntactical way. In the following call, x does not match any of the numbers 1, 2, 3 syntactically:

{1, 2, 3} intersect {1, x} Example 2

intersect is overloaded by the domain Dom::Interval:

Dom::Interval(2, infinity) intersect Dom::Interval([1, 3]) {PI/2, 2, 2.5, 3} intersect Dom::Interval(1,3) In contrast to finite sets of type DOM_SET, the interval domain works semantically. It takes properties into account:

Dom::Interval(-1, 1) intersect {x} assume(0 < x < 1):
Dom::Interval(-1, 1) intersect {x} unassume(x):

Example 3

The following list provides a collection of sets:

L := [{a, b}, {1, 2, a, c}, {3, a, b}, {a, c}]:

The functional equivalent _intersect of the intersect operator accepts an arbitrary number of arguments. Thus, the intersection of all sets in L can be computed as follows:

_intersect(op(L)) delete L:

Example 4

universe represents the set of all mathematical objects:

_intersect() Parameters

 set1, set2, … Finite sets of type DOM_SET, or intervals of type Dom::Interval, or arithmetical expressions

Return Values

Set, an interval, a symbolic expression of type "_intersect", or universe.