# Difference between revisions of "Complex"

### From Online Dictionary of Crystallography

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==Laws of composition for complexes== | ==Laws of composition for complexes== | ||

− | There exist two laws of | + | There exist two laws of composition for complexes. |

− | #'''Addition'''. The sum of two complexes K and L consists of all the elements of K and L combined. The addition of complexes is therefore a union from a set-theoretic viewpoint. It is commutative and associative. | + | #'''Addition'''. The sum of two complexes ''K'' and ''L'' consists of all the elements of ''K'' and ''L'' combined. The addition of complexes is therefore a union from a set-theoretic viewpoint. It is commutative and associative. |

− | #'''Multiplication'''. The product of two complexes K and L is the complex obtained by formal expansion: {K<sub>i</sub>L<sub>j</sub>}. It is, in general, non-commutative, but associative and distributive. | + | #'''Multiplication'''. The product of two complexes ''K'' and ''L'' is the complex obtained by formal expansion: {''K<sub>i</sub>L<sub>j</sub>''}. It is, in general, non-commutative, but associative and distributive. |

− | It is, in general, not permissible to apply the cancelling rule to complexes. This means that from the equation KL = KM does '''not''' follow that | + | It is, in general, not permissible to apply the cancelling rule to complexes. This means that from the equation ''KL = KM'' does '''not''' follow that ''L = M'', unless ''K'' is a single element. |

[[Category:Fundamental crystallography]] | [[Category:Fundamental crystallography]] |

## Revision as of 10:38, 13 May 2017

## Definition

A **complex** is a subset obtained from a group by choosing part of its elements in such a way that the closure property of groups is not respected. Therefore, a complex is not a group itself.

A typical example of complexes is that of cosets. In fact, a coset does not contain the identity and therefore it is not a group.

A subgroup is a particular case of complex that obeys the closure property and is a group itself.

## Laws of composition for complexes

There exist two laws of composition for complexes.

**Addition**. The sum of two complexes*K*and*L*consists of all the elements of*K*and*L*combined. The addition of complexes is therefore a union from a set-theoretic viewpoint. It is commutative and associative.**Multiplication**. The product of two complexes*K*and*L*is the complex obtained by formal expansion: {*K*}. It is, in general, non-commutative, but associative and distributive._{i}L_{j}

It is, in general, not permissible to apply the cancelling rule to complexes. This means that from the equation *KL = KM* does **not** follow that *L = M*, unless *K* is a single element.