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55 Additive Magmas
55.1 (Near-)Additive Magma Categories
55.1-1 IsNearAdditiveMagma
55.1-2 IsNearAdditiveMagmaWithZero
55.1-3 IsNearAdditiveGroup
55.1-4 IsAdditiveMagma
55.1-5 IsAdditiveMagmaWithZero
55.1-6 IsAdditiveGroup
55.2 (Near-)Additive Magma Generation
55.2-1 NearAdditiveMagma
55.2-2 NearAdditiveMagmaWithZero
55.2-3 NearAdditiveGroup
55.2-4 NearAdditiveMagmaByGenerators
55.2-5 NearAdditiveMagmaWithZeroByGenerators
55.2-6 NearAdditiveGroupByGenerators
55.2-7 SubnearAdditiveMagma
55.2-8 SubnearAdditiveMagmaWithZero
55.2-9 SubnearAdditiveGroup
55.3 Attributes and Properties for (Near-)Additive Magmas
55.3-1 IsAdditivelyCommutative
55.3-2 GeneratorsOfNearAdditiveMagma
55.3-3 GeneratorsOfNearAdditiveMagmaWithZero
55.3-4 GeneratorsOfNearAdditiveGroup
55.3-5 AdditiveNeutralElement
55.3-6 TrivialSubnearAdditiveMagmaWithZero
55.4 Operations for (Near-)Additive Magmas
55.4-1 ClosureNearAdditiveGroup
55.4-2 ShowAdditionTable

### 55 Additive Magmas

This chapter deals with domains that are closed under addition `+`, which are called near-additive magmas in GAP. Together with the domains closed under multiplication `*` (see 35), they are the basic algebraic structures. In many cases, the addition is commutative (see `IsAdditivelyCommutative` (55.3-1)), the domain is called an additive magma then. Every module (see 57), vector space (see 61), ring (see 56), or field (see 58) is an additive magma. In the cases of all (near-)additive magma-with-zero or (near-)additive magma-with-inverses, additional additive structure is present (see 55.1).

#### 55.1 (Near-)Additive Magma Categories

##### 55.1-1 IsNearAdditiveMagma

`‣ IsNearAdditiveMagma`( obj ) | ( category )

A near-additive magma in GAP is a domain \(A\) with an associative but not necessarily commutative addition `+`\(: A \times A \rightarrow A\).

##### 55.1-2 IsNearAdditiveMagmaWithZero

`‣ IsNearAdditiveMagmaWithZero`( obj ) | ( category )

A near-additive magma-with-zero in GAP is a near-additive magma \(A\) with an operation `0*` (or `Zero` (31.10-3)) that yields the zero element of \(A\).

So a near-additive magma-with-zero A does always contain a unique additively neutral element \(z\), i.e., \(z + a = a = a + z\) holds for all \(a \in A\) (see `AdditiveNeutralElement` (55.3-5)). This zero element \(z\) can be computed with the operation `Zero` (31.10-3), by applying this function to \(A\) or to any element \(a\) in \(A\). The zero element can be computed also as `0 *` \(a\), for any \(a\) in \(A\).

Note that it may happen that a near-additive magma containing a zero does not lie in the category `IsNearAdditiveMagmaWithZero` (see 31.6).

##### 55.1-3 IsNearAdditiveGroup

`‣ IsNearAdditiveGroup`( obj ) | ( category )

`‣ IsNearAdditiveMagmaWithInverses`( obj ) | ( category )

A near-additive group in GAP is a near-additive magma-with-zero \(A\) with an operation `-1*`\(: A \rightarrow A\) that maps each element \(a\) of \(A\) to its additive inverse `-1*`\(a\) (or `AdditiveInverse(` a`)`, see `AdditiveInverse` (31.10-9)).

The addition `+` of \(A\) is assumed to be associative, so a near-additive group is not more than a near-additive magma-with-inverses. `IsNearAdditiveMagmaWithInverses` is just a synonym for `IsNearAdditiveGroup`, and can be used alternatively in all function names involving the string `"NearAdditiveGroup"`.

Note that not every trivial near-additive magma is a near-additive magma-with-zero, but every trivial near-additive magma-with-zero is a near-additive group.

##### 55.1-4 IsAdditiveMagma

`‣ IsAdditiveMagma`( obj ) | ( category )

An additive magma in GAP is a domain \(A\) with an associative and commutative addition `+`\(: A \times A \rightarrow A\), see `IsNearAdditiveMagma` (55.1-1) and `IsAdditivelyCommutative` (55.3-1).

##### 55.1-5 IsAdditiveMagmaWithZero

`‣ IsAdditiveMagmaWithZero`( obj ) | ( category )

An additive magma-with-zero in GAP is an additive magma \(A\) (see `IsAdditiveMagma` (55.1-4) with an operation `0*` (or `Zero` (31.10-3)) that yields the zero of \(A\).

So an additive magma-with-zero \(A\) does always contain a unique additively neutral element \(z\), i.e., \(z + a = a = a + z\) holds for all \(a \in A\) (see `AdditiveNeutralElement` (55.3-5)). This element \(z\) can be computed with the operation `Zero` (31.10-3) as `Zero(` \(A\)`)`, and \(z\) is also equal to `Zero(` \(a\)`)` and to `0*`\(a\) for each element \(a\) in \(A\).

Note that it may happen that an additive magma containing a zero does not lie in the category `IsAdditiveMagmaWithZero` (see 31.6).

##### 55.1-6 IsAdditiveGroup

`‣ IsAdditiveGroup`( obj ) | ( category )

`‣ IsAdditiveMagmaWithInverses`( obj ) | ( category )

An additive group in GAP is an additive magma-with-zero \(A\) with an operation `-1*`\(: A \rightarrow A\) that maps each element \(a\) of \(A\) to its additive inverse `-1*`\(a\) (or `AdditiveInverse(` \(a\)`)`, see `AdditiveInverse` (31.10-9)).

The addition `+` of \(A\) is assumed to be commutative and associative, so an additive group is not more than an additive magma-with-inverses. `IsAdditiveMagmaWithInverses` is just a synonym for `IsAdditiveGroup`, and can be used alternatively in all function names involving the string `"AdditiveGroup"`.

Note that not every trivial additive magma is an additive magma-with-zero, but every trivial additive magma-with-zero is an additive group.

#### 55.2 (Near-)Additive Magma Generation

This section describes functions that create additive magmas from generators (see `NearAdditiveMagma` (55.2-1), `NearAdditiveMagmaWithZero` (55.2-2), `NearAdditiveGroup` (55.2-3)), the underlying operations for which methods can be installed (see `NearAdditiveMagmaByGenerators` (55.2-4), `NearAdditiveMagmaWithZeroByGenerators` (55.2-5), `NearAdditiveGroupByGenerators` (55.2-6)) and functions for forming additive submagmas (see `SubnearAdditiveMagma` (55.2-7), `SubnearAdditiveMagmaWithZero` (55.2-8), `SubnearAdditiveGroup` (55.2-9)).

##### 55.2-1 NearAdditiveMagma

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

returns the (near-)additive magma \(A\) that is generated by the elements in the list gens, that is, the closure of gens under addition `+`. The family Fam of \(A\) can be entered as first argument; this is obligatory if gens is empty (and hence also \(A\) is empty).

##### 55.2-2 NearAdditiveMagmaWithZero

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

returns the (near-)additive magma-with-zero \(A\) that is generated by the elements in the list gens, that is, the closure of gens under addition `+` and `Zero` (31.10-3). The family Fam of \(A\) can be entered as first argument; this is obligatory if gens is empty (and hence \(A\) is trivial).

##### 55.2-3 NearAdditiveGroup

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

returns the (near-)additive group \(A\) that is generated by the elements in the list gens, that is, the closure of gens under addition `+`, `Zero` (31.10-3), and `AdditiveInverse` (31.10-9). The family Fam of \(A\) can be entered as first argument; this is obligatory if gens is empty (and hence \(A\) is trivial).

##### 55.2-4 NearAdditiveMagmaByGenerators

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

An underlying operation for `NearAdditiveMagma` (55.2-1).

##### 55.2-5 NearAdditiveMagmaWithZeroByGenerators

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

An underlying operation for `NearAdditiveMagmaWithZero` (55.2-2).

##### 55.2-6 NearAdditiveGroupByGenerators

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

An underlying operation for `NearAdditiveGroup` (55.2-3).

##### 55.2-7 SubnearAdditiveMagma

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

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

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

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

`SubnearAdditiveMagma` returns the near-additive magma generated by the elements in the list gens, with parent the domain D. `SubnearAdditiveMagmaNC` does the same, except that it does not check whether the elements of gens lie in D.

`SubadditiveMagma` and `SubadditiveMagmaNC` are just synonyms of these functions.

##### 55.2-8 SubnearAdditiveMagmaWithZero

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

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

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

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

`SubnearAdditiveMagmaWithZero` returns the near-additive magma-with-zero generated by the elements in the list gens, with parent the domain D. `SubnearAdditiveMagmaWithZeroNC` does the same, except that it does not check whether the elements of gens lie in D.

`SubadditiveMagmaWithZero` and `SubadditiveMagmaWithZeroNC` are just synonyms of these functions.

##### 55.2-9 SubnearAdditiveGroup

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

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

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

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

`SubnearAdditiveGroup` returns the near-additive group generated by the elements in the list gens, with parent the domain D. `SubadditiveGroupNC` does the same, except that it does not check whether the elements of gens lie in D.

`SubadditiveGroup` and `SubadditiveGroupNC` are just synonyms of these functions.

#### 55.3 Attributes and Properties for (Near-)Additive Magmas

##### 55.3-1 IsAdditivelyCommutative

`‣ IsAdditivelyCommutative`( A ) | ( property )

A near-additive magma A in GAP is additively commutative if for all elements \(a, b \in \textit{A}\) the equality \(a + b = b + a\) holds.

Note that the commutativity of the multiplication `*` in a multiplicative structure can be tested with `IsCommutative` (35.4-9).

##### 55.3-2 GeneratorsOfNearAdditiveMagma

`‣ GeneratorsOfNearAdditiveMagma`( A ) | ( attribute )

`‣ GeneratorsOfAdditiveMagma`( A ) | ( attribute )

is a list of elements of the near-additive magma A that generates A as a near-additive magma, that is, the closure of this list under addition is A.

##### 55.3-3 GeneratorsOfNearAdditiveMagmaWithZero

`‣ GeneratorsOfNearAdditiveMagmaWithZero`( A ) | ( attribute )

`‣ GeneratorsOfAdditiveMagmaWithZero`( A ) | ( attribute )

is a list of elements of the near-additive magma-with-zero A that generates A as a near-additive magma-with-zero, that is, the closure of this list under addition and `Zero` (31.10-3) is A.

##### 55.3-4 GeneratorsOfNearAdditiveGroup

`‣ GeneratorsOfNearAdditiveGroup`( A ) | ( attribute )

`‣ GeneratorsOfAdditiveGroup`( A ) | ( attribute )

is a list of elements of the near-additive group A that generates A as a near-additive group, that is, the closure of this list under addition, taking the zero element, and taking additive inverses (see `AdditiveInverse` (31.10-9)) is A.

##### 55.3-5 AdditiveNeutralElement

`‣ AdditiveNeutralElement`( A ) | ( attribute )

returns the element \(z\) in the near-additive magma A with the property that \(z + a = a = a + z\) holds for all \(a \in\) A, if such an element exists. Otherwise `fail` is returned.

A near-additive magma that is not a near-additive magma-with-zero can have an additive neutral element \(z\); in this case, \(z\) cannot be obtained as `Zero( A )` or as `0*`\(a\) for an element \(a\) in A, see `Zero` (31.10-3).

##### 55.3-6 TrivialSubnearAdditiveMagmaWithZero

`‣ TrivialSubnearAdditiveMagmaWithZero`( A ) | ( attribute )

is the additive magma-with-zero that has the zero of the near-additive magma-with-zero A as its only element.

#### 55.4 Operations for (Near-)Additive Magmas

##### 55.4-1 ClosureNearAdditiveGroup

`‣ ClosureNearAdditiveGroup`( A, a ) | ( operation )

`‣ ClosureNearAdditiveGroup`( A, B ) | ( operation )

returns the closure of the near-additive magma A with the element a or with the near-additive magma B, w.r.t. addition, taking the zero element, and taking additive inverses.

##### 55.4-2 ShowAdditionTable

`‣ ShowAdditionTable`( R ) | ( function )

`‣ ShowMultiplicationTable`( M ) | ( function )

For a structure R with an addition given by `+`, respectively a structure M with a multiplication given by `*`, this command displays the addition (multiplication) table of the structure in a pretty way.

    gap> ShowAdditionTable(GF(4));
    +        | 0*Z(2)   Z(2)^0   Z(2^2)   Z(2^2)^2
    ---------+------------------------------------
    0*Z(2)   | 0*Z(2)   Z(2)^0   Z(2^2)   Z(2^2)^2
    Z(2)^0   | Z(2)^0   0*Z(2)   Z(2^2)^2 Z(2^2)
    Z(2^2)   | Z(2^2)   Z(2^2)^2 0*Z(2)   Z(2)^0
    Z(2^2)^2 | Z(2^2)^2 Z(2^2)   Z(2)^0   0*Z(2)

    gap> ShowMultiplicationTable(GF(4));
    *        | 0*Z(2)   Z(2)^0   Z(2^2)   Z(2^2)^2
    ---------+------------------------------------
    0*Z(2)   | 0*Z(2)   0*Z(2)   0*Z(2)   0*Z(2)
    Z(2)^0   | 0*Z(2)   Z(2)^0   Z(2^2)   Z(2^2)^2
    Z(2^2)   | 0*Z(2)   Z(2^2)   Z(2^2)^2 Z(2)^0
    Z(2^2)^2 | 0*Z(2)   Z(2^2)^2 Z(2)^0   Z(2^2)

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