# Direct Sums

AbstractAlgebra allows the construction of the external direct sum of any nonempty vector of finitely presented modules.

Note that external direct sums are considered equal iff they are the same object.

## Generic direct sum type

AbstractAlgebra provides a generic direct sum type Generic.DirectSumModule{T} where T is the element type of the base ring. The implementation is in src/generic/DirectSum.jl

Elements of direct sum modules have type Generic.DirectSumModuleElem{T}.

## Abstract types

Direct sum module types belong to the abstract type FPModule{T} and their elements to FPModuleElem{T}.

## Constructors

AbstractAlgebra.direct_sumFunction
direct_sum(m::Vector{<:FPModule{T}}) where T <: RingElement
direct_sum(vals::FPModule{T}...) where T <: RingElement

Return a tuple $M, f, g$ consisting of $M$ the direct sum of the modules m (supplied as a vector of modules), a vector $f$ of the injections of the $m[i]$ into $M$ and a vector $g$ of the projections from $M$ onto the $m[i]$.

source

Examples

julia> F = free_module(ZZ, 5)
Free module of rank 5 over integers

julia> m1 = F(BigInt[4, 7, 8, 2, 6])
(4, 7, 8, 2, 6)

julia> m2 = F(BigInt[9, 7, -2, 2, -4])
(9, 7, -2, 2, -4)

julia> S1, f1 = sub(F, [m1, m2])
(Submodule over integers with 2 generators and no relations, Hom: submodule over integers with 2 generators and no relations -> free module of rank 5 over integers)

julia> m1 = F(BigInt[3, 1, 7, 7, -7])
(3, 1, 7, 7, -7)

julia> m2 = F(BigInt[-8, 6, 10, -1, 1])
(-8, 6, 10, -1, 1)

julia> S2, f2 = sub(F, [m1, m2])
(Submodule over integers with 2 generators and no relations, Hom: submodule over integers with 2 generators and no relations -> free module of rank 5 over integers)

julia> m1 = F(BigInt[2, 4, 2, -3, -10])
(2, 4, 2, -3, -10)

julia> m2 = F(BigInt[5, 7, -6, 9, -5])
(5, 7, -6, 9, -5)

julia> S3, f3 = sub(F, [m1, m2])
(Submodule over integers with 2 generators and no relations, Hom: submodule over integers with 2 generators and no relations -> free module of rank 5 over integers)

julia> D, f = direct_sum(S1, S2, S3)
(DirectSumModule over integers, AbstractAlgebra.Generic.ModuleHomomorphism{BigInt}[Hom: submodule over integers with 2 generators and no relations -> DirectSumModule, Hom: submodule over integers with 2 generators and no relations -> DirectSumModule, Hom: submodule over integers with 2 generators and no relations -> DirectSumModule], AbstractAlgebra.Generic.ModuleHomomorphism{BigInt}[Hom: DirectSumModule -> submodule over integers with 2 generators and no relations, Hom: DirectSumModule -> submodule over integers with 2 generators and no relations, Hom: DirectSumModule -> submodule over integers with 2 generators and no relations])

## Functionality for direct sums

In addition to the Module interface, AbstractAlgebra direct sums implement the following functionality.

### Basic manipulation

Examples

julia> F = free_module(ZZ, 5)
Free module of rank 5 over integers

julia> m1 = F(BigInt[4, 7, 8, 2, 6])
(4, 7, 8, 2, 6)

julia> m2 = F(BigInt[9, 7, -2, 2, -4])
(9, 7, -2, 2, -4)

julia> S1, f1 = sub(F, [m1, m2])
(Submodule over integers with 2 generators and no relations, Hom: submodule over integers with 2 generators and no relations -> free module of rank 5 over integers)

julia> m1 = F(BigInt[3, 1, 7, 7, -7])
(3, 1, 7, 7, -7)

julia> m2 = F(BigInt[-8, 6, 10, -1, 1])
(-8, 6, 10, -1, 1)

julia> S2, f2 = sub(F, [m1, m2])
(Submodule over integers with 2 generators and no relations, Hom: submodule over integers with 2 generators and no relations -> free module of rank 5 over integers)

julia> m1 = F(BigInt[2, 4, 2, -3, -10])
(2, 4, 2, -3, -10)

julia> m2 = F(BigInt[5, 7, -6, 9, -5])
(5, 7, -6, 9, -5)

julia> S3, f3 = sub(F, [m1, m2])
(Submodule over integers with 2 generators and no relations, Hom: submodule over integers with 2 generators and no relations -> free module of rank 5 over integers)

julia> D, f = direct_sum(S1, S2, S3)
(DirectSumModule over integers, AbstractAlgebra.Generic.ModuleHomomorphism{BigInt}[Hom: submodule over integers with 2 generators and no relations -> DirectSumModule, Hom: submodule over integers with 2 generators and no relations -> DirectSumModule, Hom: submodule over integers with 2 generators and no relations -> DirectSumModule], AbstractAlgebra.Generic.ModuleHomomorphism{BigInt}[Hom: DirectSumModule -> submodule over integers with 2 generators and no relations, Hom: DirectSumModule -> submodule over integers with 2 generators and no relations, Hom: DirectSumModule -> submodule over integers with 2 generators and no relations])

julia> summands(D)
3-element Vector{AbstractAlgebra.Generic.Submodule{BigInt}}:
Submodule over integers with 2 generators and no relations
Submodule over integers with 2 generators and no relations
Submodule over integers with 2 generators and no relations
    (D::DirectSumModule{T}(::Vector{<:FPModuleElem{T}}) where T <: RingElement

Given a vector (or $1$-dim array) of module elements, where the $i$-th entry has to be an element of the $i$-summand of $D$, create the corresponding element in $D$.

Examples

julia> N = free_module(QQ, 1);

julia> M = free_module(QQ, 2);

julia> D, _ = direct_sum(M, N, M);

julia> D([gen(M, 1), gen(N, 1), gen(M, 2)])
(1//1, 0//1, 1//1, 0//1, 1//1)

### Special Homomorphisms

Due to the special structure as direct sums, homomorphisms can be created by specifying homomorphisms for all summands. In case of the codmain being a direct sum as well, any homomorphism may be thought of as a matrix containing maps from the $i$-th source summand to the $j$-th target module:

ModuleHomomorphism(D::DirectSumModule{T}, S::DirectSumModule{T}, m::Matrix{Any}) where T <: RingElement

Given a matrix $m$ such that the $(i,j)$-th entry is either $0$ (Int(0)) or a ModuleHomomorphism from the $i$-th summand of $D$ to the $j$-th summand of $S$, construct the corresponding homomorphism.

ModuleHomomorphism(D::DirectSumModule{T}, S::FPModuleElem{T}, m::Vector{ModuleHomomorphism})

Given an array $a$ of ModuleHomomorphism such that $a_i$, the $i$-th entry of $a$ is a ModuleHomomorphism from the $i$-th summand of D into S, construct the direct sum of the components.

Given a matrix $m$ such that the $(i,j)$-th entry is either $0$ (Int(0)) or a ModuleHomomorphism from the $i$-th summand of $D$ to the $j$-th summand of $S$, construct the corresponding homomorphism.

Examples

julia> N = free_module(QQ, 2);

julia> D, _ = direct_sum(N, N);

julia> p = ModuleHomomorphism(N, N, [3,4] .* basis(N));

julia> q = ModuleHomomorphism(N, N, [5,7] .* basis(N));

julia> phi = ModuleHomomorphism(D, D, [p 0; 0 q])
Module homomorphism
from DirectSumModule over rationals
to DirectSumModule over rationals

julia> r = ModuleHomomorphism(N, D, [2,3] .* gens(D)[1:2])
Module homomorphism
from vector space of dimension 2 over rationals
to DirectSumModule over rationals

julia> psi = ModuleHomomorphism(D, D, [r, r])
Module homomorphism
from DirectSumModule over rationals
to DirectSumModule over rationals