# Subgroup property modifier

*This article is about the notion of property modifier for the [[{{{1}}} property space]]: it inputs a [[{{{1}}} property]] and outputs a [[{{{1}}} property]]*

This article is about a general term. A list of important particular cases (instances) is available at Category:Subgroup property modifiers

## Definition

### Symbol-free definition

A **subgroup property modifier** is a function from the subgroup property space to itself that takes as input a subgroup property and outputs a subgroup property.

## Examples

An example of a subgroup property modifier is the subordination operator, which takes as input a subgroup property , and outputs the property of being a subgroup, which can be connected to the whole group via a series of intermediate subgroups, each having property in the next. This is the Kleene-star closure with respect to the composition operator for subgroup properties.

Another example is the left transiter of a subgroup property. The left transiter of a subgroup property is the following subgroup property : has property in if whenever such that has property in , also has property in .

## Related notions

Given a subgroup property modifier, we are often interested in the *image space* of the modifier: the collection of those subgroup properties that can be obtained by applying the modifier to some property. We are also interested in the *fixed-point space*: the collection of subgroup properties that remain unchanged on applying the modifier.

There are some subgroup property modifiers whose image space is the same as the fixed-point space; this is equivalent to the condition that applying the modifer twice has the same effect as applying it once. Such a subgroup property modifier is termed an idempotent subgroup property modifier.

There are other properties that *nice* subgroup property modifiers have. Check out: