# Help:Property-theoretic lookup

The Group Properties Wiki is designed and organized using a property-theoretic paradigm. This article sketches briefly how to use the property-theoretic paradigm to look up facts in the wiki.

## Contents

## So what's a property and what's the property-theoretic paradigm?

### What is a property?

A property over a collection of objects is something which classifies those objects into *have*s and *have not*s. In other words, given an object, it should either *have* that property or *not have* that property.

Examples from elementary mathematics:

- Being prime is a
*property*over the collection of natural numbers. In other words, every natural number either*has*the property of being prime, or*does not have*the property of being prime. - Being positive is a
*property*over the collection of real numbers. In other words, every real number either*has*the property of being positive, or*does not have*the property of being positive.

Examples from group theory:

- Being solvable is a
*property*over the collection of groups. In other words, every group either*has*the property of being solvable, or does*not*have the property of being solvable (every group is either solvable or not). - Being normal is a
*property*over the collection of subgroups. In other words, given a group and a subgroup of the group, the subgroup either*has*the property of being normal, or*does not*have the property of being normal. - Being inner is a
*property*over the collection of automorphisms. In other words, given a group and an automorphism of the group, the automorphism either*has*the property of being inner, or*does not*have the property of being inner.

`Further information: Property`

Given two properties over the same context space (i.e. two properties that are being used for the same collection of objects) we can ask questions like:

- Does the first property imply the second? For instance, is every odd number prime? (No).
- What do we get when we take the
*conjunction*of the properties. A conjunction of two properties is the AND of the properties. For instance, the property of being an odd prime is the conjunction of the property of being odd and the property of being prime.

### Metaproperties: properties of properties

A *metaproperty* is a property that we can evaluate for properties over a certain context space. For instance:

- We can ask whether a property of natural numbers is additively closed. A property of natural numbers is additively closed if whenever satisfy the property, so does . Thus, being additively closed is a metaproperty of natural numbers.
- We can ask whether a property of groups is subgroup-closed. A property of groups is termed subgroup-closed if any subgroup of a group with the property also has the property.
- We can ask whether a property of subgroups is transitive. A subgroup property is transitive if whenever are groups, such that satisfies in and satisfies in , then satisfies in .

### Property modifiers and operators

Starting with a property, we can apply a *modifier* or *operator* to it. For instance, given a group property, we can apply the hereditarily operator to it: this converts a property to the property of being a group in which *every* subgroup has property .

A property *modifier* is a property operator that takes a property and outputs another property on the *same* context space (i.e. the same collection of objects). The hereditarily operator is a property modifier. There is a more general notion of property operator, which takes in inputs as properties (possibly multiple inputs from different property spaces) and outputs a property (possibly over a different property space). Here's one example. Given a group property and a subgroup property , we can define the property of being a group in which every subgroup with *subgroup* property , satisfies group property as an abstract group.

## Property theory for definition lookup

Most definitions related to properties come under the following headers. The categories for these headers are *super*categories: they do not themselves list the properties, but rather, list the context spaces over which the properties are being evaluated.

- Category:Properties: This lists properties over various context spaces. For instance, if you're looking for solvable group, then the correct subcategory to look in would be Category:Group properties.
- Category:Metaproperties: This lists metaproperties over various context spaces. For instance, if you're looking for transitive subgroup property, the correct place to look in would be Category:Subgroup properties.
- Category:Property modifiers: This lists property operators from one property space to itself. Thus, if you're looking for an operator that takes as input a group property, tweaks it and outputs a group property, the place to look would be Category:Group property modifiers.
- Category:Property operators: This lists more general property operators, which might possibly take more than one input.

Many of the individual categories have their own lookup pages.

## Property theory for fact lookup

Most facts related to properties come under the following headers. The categories for these headers are *super*categories: they do not themselves list the properties, but rather, list the context spaces:

- Category:Property implications: Suppose you want to prove that any nilpotent group is solvable. This is a
*property implication*for groups: one group property implying the other. The correct place to look for it is Category:Group property implications. - Category:Property non-implications: If you're looking for a fact which states that a certain property does
*not*imply the other, this is where to look. For instance, to prove that not every normal subgroup is characteristic, you need to check in at Category:Subgroup property non-implications - Category:Metaproperty satisfactions: This is for facts about a property satisfying a metaproperty. For instance, characteristicity is transitive describes how a subgroup property (the property of being a characteristic subgroup) satisfies a subgroup metaproperty (the metaproperty of being a transitive subgroup property). So this will be under Category:Subgroup metaproperty satisfactions
- Category:Metaproperty dissatisfactions: This is for facts about a property
*not*satisfying a metaproperty. - [[:Category:Proper