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34 Commutative diagrams and abstract categories
 34.1  
 34.2  

34 Commutative diagrams and abstract categories

COMMUTATIVE DIAGRAMS

34.1  

34.1-1 HomomorphismChainToCommutativeDiagram
‣ HomomorphismChainToCommutativeDiagram( H )( function )

Inputs a list H=[h_1,h_2,...,h_n] of mappings such that the composite h_1h_2...h_n is defined. It returns the list of composable homomorphism as a commutative diagram.

Examples:

34.1-2 NormalSeriesToQuotientDiagram
‣ NormalSeriesToQuotientDiagram( L )( function )
‣ NormalSeriesToQuotientDiagram( L, M )( function )

Inputs an increasing (or decreasing) list L=[L_1,L_2,...,L_n] of normal subgroups of a group G with G=L_n. It returns the chain of quotient homomorphisms G/L_i → G/L_i+1 as a commutative diagram.

Optionally a subseries M of L can be entered as a second variable. Then the resulting diagram of quotient groups has two rows of horizontal arrows and one row of vertical arrows.

Examples:

34.1-3 NerveOfCommutativeDiagram
‣ NerveOfCommutativeDiagram( D )( function )

Inputs a commutative diagram D and returns the commutative diagram ND consisting of all possible composites of the arrows in D.

Examples:

34.1-4 GroupHomologyOfCommutativeDiagram
‣ GroupHomologyOfCommutativeDiagram( D, n )( function )
‣ GroupHomologyOfCommutativeDiagram( D, n, prime )( function )
‣ GroupHomologyOfCommutativeDiagram( D, n, prime, Resolution_Algorithm )( function )

Inputs a commutative diagram D of p-groups and positive integer n. It returns the commutative diagram of vector spaces obtained by applying mod p homology.

Non-prime power groups can also be handled if a prime p is entered as the third argument. Integral homology can be obtained by setting p=0. For p=0 the result is a diagram of groups.

A particular resolution algorithm, such as ResolutionNilpotentGroup, can be entered as a fourth argument. For positive p the default is ResolutionPrimePowerGroup. For p=0 the default is ResolutionFiniteGroup.

Examples:

34.1-5 PersistentHomologyOfCommutativeDiagramOfPGroups
‣ PersistentHomologyOfCommutativeDiagramOfPGroups( D, n )( function )

Inputs a commutative diagram D of finite p-groups and a positive integer n. It returns a list containing, for each homomorphism in the nerve of D, a triple [k,l,m] where k is the dimension of the source of the induced mod p homology map in degree n, l is the dimension of the image, and m is the dimension of the cokernel.

Examples:

ABSTRACT CATEGORIES

34.2  

34.2-1 CategoricalEnrichment
‣ CategoricalEnrichment( X, Name )( function )

Inputs a structure X such as a group or group homomorphism, together with the name of some existing category such as Name:=Category_of_Groups or Category_of_Abelian_Groups. It returns, as appropriate, an object or arrow in the named category.

Examples: 1 

34.2-2 IdentityArrow
‣ IdentityArrow( X )( function )

Inputs an object X in some category, and returns the identity arrow on the object X.

Examples: 1 

34.2-3 InitialArrow
‣ InitialArrow( X )( function )

Inputs an object X in some category, and returns the arrow from the initial object in the category to X.

Examples: 1 

34.2-4 TerminalArrow
‣ TerminalArrow( X )( function )

Inputs an object X in some category, and returns the arrow from X to the terminal object in the category.

Examples: 1 

34.2-5 HasInitialObject
‣ HasInitialObject( Name )( function )

Inputs the name of a category and returns true or false depending on whether the category has an initial object.

Examples: 1 

34.2-6 HasTerminalObject
‣ HasTerminalObject( Name )( function )

Inputs the name of a category and returns true or false depending on whether the category has a terminal object.

Examples:

34.2-7 Source
‣ Source( f )( function )

Inputs an arrow f in some category, and returns its source.

Examples: 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 

34.2-8 Target
‣ Target( f )( function )

Inputs an arrow f in some category, and returns its target.

Examples: 1 , 2 , 3 , 4 , 5 , 6 

34.2-9 CategoryName
‣ CategoryName( X )( function )

Inputs an object or arrow X in some category, and returns the name of the category.

Examples: 1 

34.2-10 CompositionEqualityAdditionMinus
‣ CompositionEqualityAdditionMinus( global variable )

Composition of suitable arrows f,g is given by f*g when the source of f equals the target of g. (Warning: this differes to the standard GAP convention.)

Equality is tested using f=g.

In an additive category the sum and difference of suitable arrows is given by f+g and f-g.

Examples:

34.2-11 Object
‣ Object( X )( function )

Inputs an object X in some category, and returns the GAP structure Y such that X=CategoricalEnrichment(Y,CategoryName(X)).

Examples: 1 

34.2-12 Mapping
‣ Mapping( X )( function )

Inputs an arrow f in some category, and returns the GAP structure Y such that f=CategoricalEnrichment(Y,CategoryName(X)).

Examples: 1 , 2 , 3 

34.2-13 IsCategoryObject
‣ IsCategoryObject( X )( function )

Inputs X and returns true if X is an object in some category.

Examples:

34.2-14 IsCategoryArrow
‣ IsCategoryArrow( X )( function )

Inputs X and returns true if X is an arrow in some category.

Examples:

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