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35 Magmas

35.2 Magma Generation

35.2-1 Magma

35.2-2 MagmaWithOne

35.2-3 MagmaWithInverses

35.2-4 MagmaByGenerators

35.2-5 MagmaWithOneByGenerators

35.2-6 MagmaWithInversesByGenerators

35.2-7 Submagma

35.2-8 SubmagmaWithOne

35.2-9 SubmagmaWithInverses

35.2-10 AsMagma

35.2-11 AsSubmagma

35.2-12 IsMagmaWithZeroAdjoined

35.2-13 InjectionZeroMagma

35.2-14 UnderlyingInjectionZeroMagma

35.2-1 Magma

35.2-2 MagmaWithOne

35.2-3 MagmaWithInverses

35.2-4 MagmaByGenerators

35.2-5 MagmaWithOneByGenerators

35.2-6 MagmaWithInversesByGenerators

35.2-7 Submagma

35.2-8 SubmagmaWithOne

35.2-9 SubmagmaWithInverses

35.2-10 AsMagma

35.2-11 AsSubmagma

35.2-12 IsMagmaWithZeroAdjoined

35.2-13 InjectionZeroMagma

35.2-14 UnderlyingInjectionZeroMagma

35.4 Attributes and Properties for Magmas

35.4-1 GeneratorsOfMagma

35.4-2 GeneratorsOfMagmaWithOne

35.4-3 GeneratorsOfMagmaWithInverses

35.4-4 Centralizer

35.4-5 Centre

35.4-6 Idempotents

35.4-7 IsAssociative

35.4-8 IsCentral

35.4-9 IsCommutative

35.4-10 MultiplicativeNeutralElement

35.4-11 MultiplicativeZero

35.4-12 SquareRoots

35.4-13 TrivialSubmagmaWithOne

35.4-1 GeneratorsOfMagma

35.4-2 GeneratorsOfMagmaWithOne

35.4-3 GeneratorsOfMagmaWithInverses

35.4-4 Centralizer

35.4-5 Centre

35.4-6 Idempotents

35.4-7 IsAssociative

35.4-8 IsCentral

35.4-9 IsCommutative

35.4-10 MultiplicativeNeutralElement

35.4-11 MultiplicativeZero

35.4-12 SquareRoots

35.4-13 TrivialSubmagmaWithOne

This chapter deals with domains (see?31) that are closed under multiplication `*`

. Following?[Bou70], we call them *magmas* in **GAP**. Together with the domains closed under addition `+`

(see?55), they are the basic algebraic structures; every semigroup, monoid (see?51), group (see?39), ring (see?56), or field (see?58) is a magma. In the cases of a *magma-with-one* or *magma-with-inverses*, additional multiplicative structure is present, see?35.1. For functions to create free magmas, see?36.4.

`‣ IsMagma` ( obj ) | ( category ) |

A *magma* in **GAP** is a domain M with (not necessarily associative) multiplication `*`

: M × M → M.

`‣ IsMagmaWithOne` ( obj ) | ( category ) |

A *magma-with-one* in **GAP** is a magma M with an operation `^0`

(or `One`

(31.10-2)) that yields the identity of M.

So a magma-with-one M does always contain a unique multiplicatively neutral element e, i.e., e` * `

m = m = m` * `

e holds for all m ∈ M (see?`MultiplicativeNeutralElement`

(35.4-10)). This element e can be computed with the operation `One`

(31.10-2) as `One( `

M` )`

, and e is also equal to `One( `

m` )`

and to m`^0`

for each element m ∈ M.

*Note* that a magma may contain a multiplicatively neutral element but *not* an identity (see?`One`

(31.10-2)), and a magma containing an identity may *not* lie in the category `IsMagmaWithOne`

(see Section?31.6).

`‣ IsMagmaWithInversesIfNonzero` ( obj ) | ( category ) |

An object in this **GAP** category is a magma-with-one M with an operation `^-1`

: M ? Z → M ? Z that maps each element m of M ? Z to its inverse m`^-1`

(or `Inverse( `

m` )`

, see?`Inverse`

(31.10-8)), where Z is either empty or consists exactly of one element of M.

This category was introduced mainly to describe division rings, since the nonzero elements in a division ring form a group; So an object M in `IsMagmaWithInversesIfNonzero`

will usually have both a multiplicative and an additive structure (see?55), and the set Z, if it is nonempty, contains exactly the zero element (see?`Zero`

(31.10-3)) of M.

`‣ IsMagmaWithInverses` ( obj ) | ( category ) |

A *magma-with-inverses* in **GAP** is a magma-with-one M with an operation `^-1`

: M → M that maps each element m of M to its inverse m`^-1`

(or `Inverse( `

m` )`

, see?`Inverse`

(31.10-8)).

Note that not every trivial magma is a magma-with-one, but every trivial magma-with-one is a magma-with-inverses. This holds also if the identity of the magma-with-one is a zero element. So a magma-with-inverses-if-nonzero can be a magma-with-inverses if either it contains no zero element or consists of a zero element that has itself as zero-th power.

This section describes functions that create magmas from generators (see `Magma`

(35.2-1), `MagmaWithOne`

(35.2-2), `MagmaWithInverses`

(35.2-3)), the underlying operations for which methods can be installed (see `MagmaByGenerators`

(35.2-4), `MagmaWithOneByGenerators`

(35.2-5), `MagmaWithInversesByGenerators`

(35.2-6)), functions for forming submagmas (see `Submagma`

(35.2-7), `SubmagmaWithOne`

(35.2-8), `SubmagmaWithInverses`

(35.2-9)), and functions that form a magma equal to a given collection (see `AsMagma`

(35.2-10), `AsSubmagma`

(35.2-11)).

`InjectionZeroMagma`

(35.2-13) creates a new magma which is the original magma with a zero adjoined.

`‣ Magma` ( [Fam, ]gens ) | ( function ) |

returns the magma M that is generated by the elements in the list `gens`, that is, the closure of `gens` under multiplication `\*`

(31.12-1). The family `Fam` of M can be entered as the first argument; this is obligatory if `gens` is empty (and hence also M is empty).

`‣ MagmaWithOne` ( [Fam, ]gens ) | ( function ) |

returns the magma-with-one M that is generated by the elements in the list `gens`, that is, the closure of `gens` under multiplication `\*`

(31.12-1) and `One`

(31.10-2). The family `Fam` of M can be entered as first argument; this is obligatory if `gens` is empty (and hence M is trivial).

`‣ MagmaWithInverses` ( [Fam, ]gens ) | ( function ) |

returns the magma-with-inverses M that is generated by the elements in the list `gens`, that is, the closure of `gens` under multiplication `\*`

(31.12-1), `One`

(31.10-2), and `Inverse`

(31.10-8). The family `Fam` of M can be entered as first argument; this is obligatory if `gens` is empty (and hence M is trivial).

`‣ MagmaByGenerators` ( [Fam, ]gens ) | ( operation ) |

An underlying operation for `Magma`

(35.2-1).

`‣ MagmaWithOneByGenerators` ( [Fam, ]gens ) | ( operation ) |

An underlying operation for `MagmaWithOne`

(35.2-2).

`‣ MagmaWithInversesByGenerators` ( [Fam, ]gens ) | ( operation ) |

An underlying operation for `MagmaWithInverses`

(35.2-3).

`‣ Submagma` ( D, gens ) | ( function ) |

`‣ SubmagmaNC` ( D, gens ) | ( function ) |

`Submagma`

returns the magma generated by the elements in the list `gens`, with parent the domain `D`. `SubmagmaNC`

does the same, except that it is not checked whether the elements of `gens` lie in `D`.

`‣ SubmagmaWithOne` ( D, gens ) | ( function ) |

`‣ SubmagmaWithOneNC` ( D, gens ) | ( function ) |

`SubmagmaWithOne`

returns the magma-with-one generated by the elements in the list `gens`, with parent the domain `D`. `SubmagmaWithOneNC`

does the same, except that it is not checked whether the elements of `gens` lie in `D`.

`‣ SubmagmaWithInverses` ( D, gens ) | ( function ) |

`‣ SubmagmaWithInversesNC` ( D, gens ) | ( function ) |

`SubmagmaWithInverses`

returns the magma-with-inverses generated by the elements in the list `gens`, with parent the domain `D`. `SubmagmaWithInversesNC`

does the same, except that it is not checked whether the elements of `gens` lie in `D`.

`‣ AsMagma` ( C ) | ( attribute ) |

For a collection `C` whose elements form a magma, `AsMagma`

returns this magma. Otherwise `fail`

is returned.

`‣ AsSubmagma` ( D, C ) | ( operation ) |

Let `D` be a domain and `C` a collection. If `C` is a subset of `D` that forms a magma then `AsSubmagma`

returns this magma, with parent `D`. Otherwise `fail`

is returned.

`‣ IsMagmaWithZeroAdjoined` ( M ) | ( category ) |

Returns: `true`

or `false`

.

`IsMagmaWithZeroAdjoined`

returns `true`

if the magma `M` was created using `InjectionZeroMagma`

(35.2-13) or `MagmaWithZeroAdjoined`

(35.2-13) and returns `false`

if it was not.

gap> S:=Semigroup(Transformation([1,1,1]), Transformation([1,3,2]));; gap> IsMagmaWithZeroAdjoined(S); false gap> M:=MagmaWithZeroAdjoined(S); <<transformation semigroup of degree 3 with 2 generators> with 0 adjoined> gap> IsMagmaWithZeroAdjoined(M); true

`‣ InjectionZeroMagma` ( M ) | ( attribute ) |

`‣ MagmaWithZeroAdjoined` ( M ) | ( attribute ) |

`InjectionZeroMagma`

returns an embedding from the magma `M` into a new magma formed from `M` by adjoining a single new element which is the multiplicative zero of the resulting magma. The elements of the new magma form a family of elements in the category `IsMultiplicativeElementWithZero`

(31.14-12) and the magma itself satisfies `IsMagmaWithZeroAdjoined`

(35.2-12).

`MagmaWithZeroAdjoined`

is just shorthand for `Range(InjectionZeroMagma(`

.`M`)))

If `N`

is a magma with zero adjoined, then the embedding used to create `N`

can be recovered using `UnderlyingInjectionZeroMagma`

(35.2-14).

gap> S:=Monoid(Transformation( [ 7, 7, 5, 3, 1, 3, 7 ] ), > Transformation( [ 5, 1, 4, 1, 4, 4, 7 ] ));; gap> MultiplicativeZero(S); Transformation( [ 7, 7, 7, 7, 7, 7, 7 ] ) gap> T:=MagmaWithZeroAdjoined(S); <<transformation monoid of degree 7 with 2 generators> with 0 adjoined> gap> map:=UnderlyingInjectionZeroMagma(T);; gap> x:=Transformation( [ 7, 7, 7, 3, 7, 3, 7 ] );; gap> x^map; <monoid with 0 adjoined elt: Transformation( [ 7, 7, 7, 3, 7, 3, 7 ] )> gap> PreImage(map, x^map)=x; true

`‣ UnderlyingInjectionZeroMagma` ( M ) | ( attribute ) |

`UnderlyingInjectionZeroMagma`

returns the embedding used to create the magma with zero adjoined `M`.

gap> S:=Monoid(Transformation( [ 8, 7, 5, 3, 1, 3, 8, 8 ] ), > Transformation( [ 5, 1, 4, 1, 4, 4, 7, 8 ] ));; gap> MultiplicativeZero(S); Transformation( [ 8, 8, 8, 8, 8, 8, 8, 8 ] ) gap> T:=MagmaWithZeroAdjoined(S); <<transformation monoid of degree 8 with 2 generators> with 0 adjoined> gap> UnderlyingInjectionZeroMagma(T); MappingByFunction( <transformation monoid of degree 8 with 2 generators>, <<transformation monoid of degree 8 with 2 generators> with 0 adjoined>, function( elt ) ... end, function( x ) ... end )

The most elementary (but of course usually not recommended) way to implement a magma with only few elements is via a multiplication table.

`‣ MagmaByMultiplicationTable` ( A ) | ( function ) |

For a square matrix `A` with n rows such that all entries of `A` are in the range [ 1 .. n ], `MagmaByMultiplicationTable`

returns a magma M with multiplication `*`

defined by `A`. That is, M consists of the elements m_1, m_2, ..., m_n, and m_i * m_j = m_k, with k = `A`[i][j].

The ordering of elements is defined by m_1 < m_2 < ? < m_n, so m_i can be accessed as `MagmaElement( `

, see?`M`, `i` )`MagmaElement`

(35.3-4).

gap> MagmaByMultiplicationTable([[1,2,3],[2,3,1],[1,1,1]]); <magma with 3 generators>

`‣ MagmaWithOneByMultiplicationTable` ( A ) | ( function ) |

The only differences between `MagmaByMultiplicationTable`

(35.3-1) and `MagmaWithOneByMultiplicationTable`

are that the latter returns a magma-with-one (see?`MagmaWithOne`

(35.2-2)) if the magma described by the matrix `A` has an identity, and returns `fail`

if not.

gap> MagmaWithOneByMultiplicationTable([[1,2,3],[2,3,1],[3,1,1]]); <magma-with-one with 3 generators> gap> MagmaWithOneByMultiplicationTable([[1,2,3],[2,3,1],[1,1,1]]); fail

`‣ MagmaWithInversesByMultiplicationTable` ( A ) | ( function ) |

`MagmaByMultiplicationTable`

(35.3-1) and `MagmaWithInversesByMultiplicationTable`

differ only in that the latter returns magma-with-inverses (see?`MagmaWithInverses`

(35.2-3)) if each element in the magma described by the matrix `A` has an inverse, and returns `fail`

if not.

gap> MagmaWithInversesByMultiplicationTable([[1,2,3],[2,3,1],[3,1,2]]); <magma-with-inverses with 3 generators> gap> MagmaWithInversesByMultiplicationTable([[1,2,3],[2,3,1],[3,2,1]]); fail

`‣ MagmaElement` ( M, i ) | ( function ) |

For a magma `M` and a positive integer `i`, `MagmaElement`

returns the `i`-th element of `M`, w.r.t.?the ordering `<`

. If `M` has less than `i` elements then `fail`

is returned.

`‣ MultiplicationTable` ( elms ) | ( attribute ) |

`‣ MultiplicationTable` ( M ) | ( attribute ) |

For a list `elms` of elements that form a magma M, `MultiplicationTable`

returns a square matrix A of positive integers such that A[i][j] = k holds if and only if `elms`[i] * `elms`[j] = `elms`[k]. This matrix can be used to construct a magma isomorphic to M, using `MagmaByMultiplicationTable`

(35.3-1).

For a magma `M`, `MultiplicationTable`

returns the multiplication table w.r.t.?the sorted list of elements of `M`.

gap> l:= [ (), (1,2)(3,4), (1,3)(2,4), (1,4)(2,3) ];; gap> a:= MultiplicationTable( l ); [ [ 1, 2, 3, 4 ], [ 2, 1, 4, 3 ], [ 3, 4, 1, 2 ], [ 4, 3, 2, 1 ] ] gap> m:= MagmaByMultiplicationTable( a ); <magma with 4 generators> gap> One( m ); m1 gap> elm:= MagmaElement( m, 2 ); One( elm ); elm^2; m2 m1 m1 gap> Inverse( elm ); m2 gap> AsGroup( m ); <group of size 4 with 2 generators> gap> a:= [ [ 1, 2 ], [ 2, 2 ] ]; [ [ 1, 2 ], [ 2, 2 ] ] gap> m:= MagmaByMultiplicationTable( a ); <magma with 2 generators> gap> One( m ); Inverse( MagmaElement( m, 2 ) ); m1 fail

*Note* that `IsAssociative`

(35.4-7) and `IsCommutative`

(35.4-9) always refer to the multiplication of a domain. If a magma `M` has also an *additive structure*, e.g., if `M` is a ring (see?56), then the addition `+`

is always assumed to be associative and commutative, see?31.12.

`‣ GeneratorsOfMagma` ( M ) | ( attribute ) |

is a list `gens` of elements of the magma `M` that generates `M` as a magma, that is, the closure of `gens` under multiplication `\*`

(31.12-1) is `M`.

For a free magma, each generator can also be accessed using the `.`

operator (see `GeneratorsOfDomain`

(31.9-2)).

`‣ GeneratorsOfMagmaWithOne` ( M ) | ( attribute ) |

is a list `gens` of elements of the magma-with-one `M` that generates `M` as a magma-with-one, that is, the closure of `gens` under multiplication `\*`

(31.12-1) and `One`

(31.10-2) is `M`.

For a free magma with one, each generator can also be accessed using the `.`

operator (see `GeneratorsOfDomain`

(31.9-2)).

`‣ GeneratorsOfMagmaWithInverses` ( M ) | ( attribute ) |

is a list `gens` of elements of the magma-with-inverses `M` that generates `M` as a magma-with-inverses, that is, the closure of `gens` under multiplication `\*`

(31.12-1) and taking inverses (see?`Inverse`

(31.10-8)) is `M`.

`‣ Centralizer` ( M, elm ) | ( operation ) |

`‣ Centralizer` ( M, S ) | ( operation ) |

`‣ Centralizer` ( class ) | ( attribute ) |

For an element `elm` of the magma `M` this operation returns the *centralizer* of `elm`. This is the domain of those elements `m` ∈ `M` that commute with `elm`.

For a submagma `S` it returns the domain of those elements that commute with *all* elements `s` of `S`.

If `class` is a class of objects of a magma (this magma then is stored as the `ActingDomain`

of `class`) such as given by `ConjugacyClass`

(39.10-1), `Centralizer`

returns the centralizer of `Representative(`

(which is a slight abuse of the notation).`class`)

gap> g:=Group((1,2,3,4),(1,2));; gap> Centralizer(g,(1,2,3)); Group([ (1,2,3) ]) gap> Centralizer(g,Subgroup(g,[(1,2,3)])); Group([ (1,2,3) ]) gap> Centralizer(g,Subgroup(g,[(1,2,3),(1,2)])); Group(())

`‣ Centre` ( M ) | ( attribute ) |

`‣ Center` ( M ) | ( attribute ) |

`Centre`

returns the *centre* of the magma `M`, i.e., the domain of those elements `m` ∈ `M` that commute and associate with all elements of `M`. That is, the set { m ∈ M; ? a, b ∈ M: ma = am, (ma)b = m(ab), (am)b = a(mb), (ab)m = a(bm) }.

`Center`

is just a synonym for `Centre`

.

For associative magmas we have that `Centre( `

, see?`M` ) = Centralizer( `M`, `M` )`Centralizer`

(35.4-4).

The centre of a magma is always commutative (see?`IsCommutative`

(35.4-9)). (When one installs a new method for `Centre`

, one should set the `IsCommutative`

(35.4-9) value of the result to `true`

, in order to make this information available.)

`‣ Idempotents` ( M ) | ( attribute ) |

The set of elements of `M` which are their own squares.

`‣ IsAssociative` ( M ) | ( property ) |

A magma `M` is *associative* if for all elements a, b, c ∈ `M` the equality (a` * `

b)` * `

c = a` * `

(b` * `

c) holds.

An associative magma is called a *semigroup* (see?51), an associative magma-with-one is called a *monoid* (see?51), and an associative magma-with-inverses is called a *group* (see?39).

`‣ IsCentral` ( M, obj ) | ( operation ) |

`IsCentral`

returns `true`

if the object `obj`, which must either be an element or a magma, commutes with all elements in the magma `M`.

`‣ IsCommutative` ( M ) | ( property ) |

`‣ IsAbelian` ( M ) | ( property ) |

A magma `M` is *commutative* if for all elements a, b ∈ `M` the equality a` * `

b = b` * `

a holds. `IsAbelian`

is a synonym of `IsCommutative`

.

Note that the commutativity of the *addition* `\+`

(31.12-1) in an additive structure can be tested with `IsAdditivelyCommutative`

(55.3-1).

`‣ MultiplicativeNeutralElement` ( M ) | ( attribute ) |

returns the element e in the magma `M` with the property that e` * `

m = m = m` * `

e holds for all m ∈ `M`, if such an element exists. Otherwise `fail`

is returned.

A magma that is not a magma-with-one can have a multiplicative neutral element e; in this case, e *cannot* be obtained as `One( `

, see?`M` )`One`

(31.10-2).

`‣ MultiplicativeZero` ( M ) | ( attribute ) |

`‣ IsMultiplicativeZero` ( M, z ) | ( operation ) |

`MultiplicativeZero`

returns the multiplicative zero of the magma `M` which is the element `z`

in `M` such that

for all `z` * `m` = `m` * `z` = `z``m` in `M`.

`IsMultiplicativeZero`

returns `true`

if the element `z` of the magma `M` equals the multiplicative zero of `M`.

gap> S:=Semigroup( Transformation( [ 1, 1, 1 ] ), > Transformation( [ 2, 3, 1 ] ) ); <transformation semigroup of degree 3 with 2 generators> gap> MultiplicativeZero(S); fail gap> S:=Semigroup( Transformation( [ 1, 1, 1 ] ), > Transformation( [ 1, 3, 2 ] ) ); <transformation semigroup of degree 3 with 2 generators> gap> MultiplicativeZero(S); Transformation( [ 1, 1, 1 ] )

`‣ SquareRoots` ( M, elm ) | ( operation ) |

is the proper set of all elements r in the magma `M` such that r * r = `elm` holds.

`‣ TrivialSubmagmaWithOne` ( M ) | ( attribute ) |

is the magma-with-one that has the identity of the magma-with-one `M` as only element.

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