Questions tagged [direct-sum]

For questions about taking the direct sum of groups and other algebraic structures.

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Does the direct sum have a universal property in the category of groups?

In other categories, like modules, the direct sum is the coproduct, giving it a neat category theory description. The direct product of groups, which is very similar to it, is the product in the ...
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1 vote
0 answers
23 views

Infinite decomposition of a Hilbert space

Let $H$ be a Hilbert space. Suppose we have $H_{n}\oplus G_{n}=H$ for all $% n\geq 0$ so that $\left( H_{n}\right) _{n\geq 0}$ is an increasing sequence of closed subspaces and $\left( G_{n}\right) _{...
0 votes
0 answers
33 views

What is the precise meaning of $c_\infty \cong c_0 \oplus \mathbb{C}$?

I am supposed to show that $c_\infty \cong c_0 \oplus \mathbb{C}$, where $$ c_\infty := \{(a_n)_{n \in \mathbb{N}}| a_n \in \mathbb{C}, a_n \to a \in \mathbb{C}\}, \\ c_0 := \{(a_n)_{n \in \mathbb{N}}|...
1 vote
1 answer
31 views

Hyperplanes question in Tu's book on Manifolds

I am trying an exercise from Tu's book "An introduction to Manifolds". Specifically, Subchapter 1.3, exercise 2 (b). I would like, before continuing with my question, to discuss something ...
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1 vote
0 answers
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When does $A^\perp = 0$ imply that $A=X$ for a subspace $A\subset X$ of a Banach space

Setup Consider the Banach space $X=\mathcal C^1([0,1],\mathbb R^n)$ with norm $$ \|f\|=\sup |f|+\sup |\dot f|. $$ This banach space has a symmetric bilinear positive definite form $$ b(f,g)=\int_0^1\...
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3 votes
1 answer
61 views

If $A$ and $A^\perp$ are closed then $A\oplus A^\perp=B$ for a Banach space $B$

Let $b$ be a bounded, positive definite and symmetric bilinear form on a Banach space $B$. I want to prove the following: Let $A\subset B$ be a closed subspace such that $A^\perp$ is closed too. Then $...
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0 votes
1 answer
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Decomposition of TTM into HM and VM

Why is it that if I have a smooth manifold and a connection map $K$, defined below, is it the case that it induces a decomposition of the tangent space to the tangent space to the manifold, given as ...
1 vote
1 answer
41 views

Are there internal direct products of vector spaces?

When I learned linear algebra I was a bit confused about the notation $A \oplus B$, the direct sum of two vector spaces $A, B$ over some field $K$. As a set, $A \oplus B$ is nothing else than the ...
2 votes
1 answer
49 views

$N_i$'s independent $\iff$ $\oplus_i N_i\cong\sum_i N_i$?

Notation and conventions: I will use $\oplus$ for the "external direct sums" only. All the modules will be over a commutative ring with unity, $A$. Problem: In the proof of one of the ...
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1 vote
1 answer
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Theorem 5, Section 7.2 of Hoffman’s Linear Algebra

Definition: $A\in M_{n\times n}(F)$ is in rational form if $$A=\begin{bmatrix} A_1& & \\ & \ddots & \\ & & A_r\\ \end{bmatrix}$$ where $A_i$ is companion matrix of non scalar ...
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1 vote
0 answers
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Direct Sum of Topological $R$-modules

Please help me. I am proving the following property: Let $R$ be a topological ring and $\{A_{\lambda}\}_{\lambda\in\Lambda}$ be a family of topological $R$-modules. Let $B_{\lambda}$ be an ...
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2 votes
1 answer
41 views

Is a group complement of a subspace of a vector space necessarily a subspace?

Let $W$ be a vector space. Let $U$ be a subspace and $V$ be a subgroup of $W$, and assume that $W$ is the direct sum (as groups!) of $U$ and $V$. Does it follow that $V$ is a subspace, such that in ...
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0 votes
1 answer
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Endomorphisms between direct product/sum of algebraic objects?

My motivation is this Wikipedia article. Suppose $R$ be a ring with unity and $M,N$ be $R$-modules. Take their direct sum/product $P=M \oplus N$. So $P$ is also a $R$-module. Consider the respective ...
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1 vote
1 answer
39 views

About the definition of direct sum of Hilbert spaces

Let $\{\mathscr{H}_{i}\}_{i\in I}$ be a family of Hilbert spaces. Their direct sum, denoted by: $$\mathscr{H} = \bigoplus_{i} \mathscr{H}_{i},$$ is defined as the space of all functions $\psi: I \to \...
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3 votes
0 answers
55 views

Exercise 11, Section 6.6 of Hoffman’s Linear Algebra

Let $V$ be a vector space, let $W_1, \ldots, W_k$ be subspaces of $V$, and let $$V_j = W_1 + \cdots + W_{j-1} + W_{j+1} + \cdots + W_k.$$ Suppose that $V = W_1 \oplus \cdots \oplus W_k$. Prove that ...
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0 votes
1 answer
42 views

What are $e_1, e_2, h_1$ and $h_2 \ ?$

Let $W$ be a finite dimensional complex vector space and $V$ and $U$ be subspaces of $W$ with $V = \mathbb C e \oplus \mathbb C h$ and $U = \mathbb C f \oplus \mathbb C h,$ for some $e,f,h \in W.$ ...
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0 votes
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Remarks of Theorem 11, Section 6.7 of Hoffman’s Linear Algebra

Following are remarks of theorem 11 section 6.7 : (i) One of the pleasant features of the decomposition $T=c_1E_1+…+c_kE_k$ is that if $g$ is any polynomial over the field $F$, then $g(T)=g(c_1)E_1+…+...
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5 votes
1 answer
64 views

The functor $\mathrm{Hom}(A,-)$ cannot commute with arbitrary direct sums for infinitely generated projective module $A$

It is easy to see the functor $\mathrm{Hom}(A,-)$ commutes with every arbitrary direct sum (i.e. $\mathrm{Hom} (A,\oplus_{i\in I} N_i)=\oplus_{i\in I}\mathrm{Hom}(A,N_i)$) for finitely generated ...
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Theorem 11, Section 6.7 of Hoffman’s Linear Algebra

Let $T$ be a linear operator on a finite-dimensional space $V$. If $T$ is diagonalizable and if $c_1,…, c_k$ are the distinct characteristic values of $T$, then there exist linear operators $E_1,…,E_k$...
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Exercise 4, Section 6.7 of Hoffman’s Linear Algebra

Let $T$ be a linear operator on $V$. Suppose $V=W_1\oplus … \oplus W_k$,where each $W_i$ is invariant under $T$. Let $T_i$ be the induced (restriction) operator on $W_i$. (a) Prove that $\det (T)=\det ...
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0 votes
1 answer
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Determine a percentage of an unfilled quota

I am having a bit of a computer programming challenge, c#, however the actual issue I believe stems from myself not understanding the maths behing it, hence me posting here... So my challenge. We have ...
0 votes
0 answers
60 views

Theorem 10, Section 6.7 of Hoffman’s Linear Algebra

Theorem 9: $V=W_1\oplus …\oplus W_k$$\iff$$\exists E_1,…,E_k\in L(V,V)$ such that (i) each $E_i$ is projection ($E_i^2=E_i$) (ii) $E_iE_j=0$, if $i\neq j$ (iii) $I=E_1+…+E_k$ (iv) $R_{E_i}=W_i$. ...
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0 votes
1 answer
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range of continous linear operator between banach spaces with closed algebraic complement

Let X, Y be Banach spaces, and let A ∈ L(X, Y ). Suppose that there exists a closed subspace W ⊂ Y so that Y is the algebraic direct sum of ran A = A(X) and W. (That is, every y ∈ Y can be written as ...
0 votes
1 answer
61 views

Exercise 2, Section 6.6 of Hoffman’s Linear Algebra

Let $V$ be a finite-dimensional vector space and let $W_1,…,W_k$ be subspaces of $V$ such that $V=W_1+…+W_k$ and $\dim(V)=\dim(W_1)+…+\dim (W_k)$. Prove that $V=W_1\oplus …\oplus W_k$. Approach (1): ...
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0 votes
0 answers
30 views

A complex involving an infinite direct sum

Let $A,B_i,C$, for $i \in I$ an infinite index set, be abelian groups such that we have group homomorphisms $$r_i: A \rightarrow B_i, \,\,\, s_i: B_i \rightarrow C$$ and a complex $$A \xrightarrow{\...
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1 vote
2 answers
33 views

Decomposing the topology of a normed linear space as the product topology of its subspaces

Let's take a normed linear space $V$ and its subspaces $U$ and $W$ such that $V = U\oplus W$. Now, $U$, $W$ are normed linear spaces in their own right. It's easily shown that $U\oplus W\cong U\times ...
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2 votes
2 answers
116 views

Linear transformation and direct sum

When given $T:V \to V$ such that $T^2=I$, prove that $U,W \subseteq V$ subspaces exist such that: $$V=U\oplus W$$ $$T(u)=u, \forall u \in U$$ $$T(w)=-w, \forall w \in W$$ My first thought was to ...
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3 votes
2 answers
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Prove that if $\dim V= n$ and $S$ is a subspace (of dimension $n-1$), then if $v$ doesn't belong to $S$: $\langle v \rangle + S$ is a direct sum

I was doing direct sum exercises and I found this problem: Assume you have a $k$-vector space $V$ of dimension $n$ and $S$ subspace (of $V$) of dimension $n-1$. I) Prove that if $v$ doesn't belong to ...
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0 votes
1 answer
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Extending a basis to make a direct sum

Let $V$ be a finite-dimensional vector space. Let $U$ be a subspace of $V$. If I extend a basis $\{v_1, ... , v_m\}$ for $U$ to a basis $\{v_1, ... , v_n\}$ for $V$, does this then imply that $V=U\...
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1 vote
1 answer
165 views

How do I prove that if $A$ and $B$ are complementary then $A^\perp$ and $B^\perp$ are also complementary in $\mathcal{H}$ Hilbert space?

I have the following problem: I know that $A$ and $B$ are two closed complementary subspaces in an Hilbert space $\mathcal{H}$, i.e. $\mathcal{H}=A\oplus B$ (where $\oplus$ is the direct sum of two ...
1 vote
1 answer
37 views

Kronecker product between a matrix and a direct sum

Is there any way to simplify this matrix expression, using something like a distributive property? $A \otimes \left( M_1 \oplus M_2 \right)$ where $\otimes$ is the Kronecker product and $\oplus$ is ...
4 votes
1 answer
137 views

Proving existence of matrix N such that M = MNM

Doing practice problems for my qualifying exam and am a bit stumped by the following: Let $n \ge 1$, $F$ be a field, and let $M$ be a matrix in $M_n(F)$. Show that there exists an $N \in M_n(F)$ such ...
1 vote
1 answer
42 views

Find a subspace $W$ of $F^4$ such that $F^4=U \oplus W$

I have to find a subspace $W$ of $F^4$ such that $F^4=U \oplus W$. $U:=\{(x,x,y,y) \in F^4:x,y \in F\}$ I found the topic with the same problem. The solution was: $W:=\{(0,b-a,c-d,0)\in F^4:a,b,c,d \...
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3 votes
0 answers
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For $K\oplus L$ if $N$ is normal in $L$ then $(K\oplus L) / N = K\oplus (L/N)$

I'm a bit new to direct sums etc., so for the purpose of convincing myself, formally, that $(\mathbb{Z} \oplus \mathbb{Z})/\langle (2,2)\rangle = \mathbb{Z} \oplus \mathbb{Z}_2$, using the basis $\{(1,...
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1 vote
1 answer
77 views

Difference between direct sum of vector spaces and direct sum of representations.

Let $G$ be a group. Let $V$ and $W$ be vector spaces which have the structures of $G$-modules. If $U \simeq V \oplus W$ as vector spaces then can't we say that they are same as $G$-modules? My teacher ...
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0 votes
0 answers
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Determine those s ∈ $\mathbb R$ for which $U_1$ + $T_s^+$ matches with $\mathbb R^\mathbb R$

For any s ∈ $\mathbb R$ the subsets of $\mathbb R^\mathbb R$ are difined: $T_s^+$ := {f ∈ $\mathbb R^\mathbb R$ | f(x) = 0 for all x $\ge$ s}, $T_s^-$ := {f ∈ $\mathbb R^\mathbb R$ | f(x) = 0 for all ...
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0 votes
0 answers
61 views

Determine bases of U1 + U2, U2 + U3, U1 + U2 + U3. Which of these sums are direct

Three subspaces of R^4×1; U1 =[a], U2 =[{b,c}], U3 =[{b, a}] a = (0, 1, 1, 0)^T , b = (1, 0, 0, 1)^T , c = (0, 1, 0, 0)^T. Determine bases of U1 + U2, U2 + U3, U1 + U2 + U3. Which of these sums are ...
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0 votes
1 answer
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Can linearity on subspaces imply linearity on their direct sum?

I read an interesting fact while learning calculus of functions of several variables: for fixed bases in $\mathbb{R}^m$ and $\mathbb{R}^n$, the linear mapping $L:\mathbb{R}^m \rightarrow \mathbb{R}^n$ ...
2 votes
1 answer
55 views

Cross-multiplication theorem of quotient modules.

We know that if $M$ is an $R$-module and $N\subset M$ is a submodule then $M/N\simeq M'$ may not imply $M\simeq N\oplus M'$.In our module theory class,it has been proved that if $M/N\simeq R^k$ then $...
0 votes
0 answers
51 views

Direct sum of two Hilbert spaces is a Hilbert space

I want to prove that the direct sum of two (complex) Hilbert spaces is a Hilbert space. I've shown that we have an inner product and also shown norm however I have trouble to show converges. We define ...
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2 votes
2 answers
68 views

I need to prove that $Ker(u - Id_E)\oplus Ker(u - aId_E)\oplus Ker(u - a^2Id_E)=E$

Let be E a $\mathbb{C}$ vectorial space and $u\in\mathcal{L}(E,E)$ a linear operator such that $u^3 = Id_E$. Prove that $$E = Ker(u - Id_E)\oplus Ker(u - aId_E)\oplus Ker(u - a^2Id_E)$$ where $a = -\...
1 vote
1 answer
73 views

Spectrum in terms of two spectra in bounded Hilbert spaces.

If we have $H_1,H_2$ as hilbert spaces, then $$H_1\oplus H_2:=\{(h,k):h\in H_1, k\in H_2\}$$ then it is also a Hilbert space with the following, $$(h_1,k_1)+(h_2,k_2):=(h_1+h_2,k_1+k_2)$$ and $$\...
0 votes
0 answers
44 views

Example to definition

A direct sum decomposition is $S=\bigoplus_{a \ge 0} S_a$ (d is degree of forms homegeneous polynomial). I try to find and example of this definition, because I need to understand it. Can I take ...
1 vote
2 answers
90 views

Spectrum of direct sum of bounded operators

I recently learned something about spectrum in functional analysis and saw some examples. However I struggling with this when trying to understand how it can be used for the following example ...
0 votes
1 answer
33 views

Using the First Isomorphism Theorem to find a linear transformation

For a vector space V with subspaces U and W, how can we find a suitable linear transformation that by the first isomorphism theorem produces (U + W)/W $\simeq$ U/(U $\displaystyle \cap$ W)? I gather ...
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0 votes
1 answer
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Confusion about the direct sum of abelian/nonabelian groups [closed]

I know that there have been many questions on this site about the relationship between the direct product and direct sum of groups. But it seems they don't address the specific issue that I want to ...
4 votes
1 answer
139 views

Norm of bounded operators of direct sum

Yesterday I posted this one regarding direct sum on Hilbert spaces $H_1$ and $H_2$ - have a look! Direct sum of two Hilbert spaces is a inner product. I am studying bounded operators and I just want ...
4 votes
0 answers
83 views

Direct sum of two Hilbert spaces is a inner product.

Today I am working with Functional Analysis regarding Hilbert Spaces from the notes. Let $H_1$ and $H_2$ be Hilbert Spaces. The direct sum $H_1\oplus H_2$ is the vector spaces $H_1\times H_2$ with the ...
0 votes
1 answer
53 views

Show that $\mathbb{R}^n$ is a direct sum of lines and planes

Let $A \in M_n(\mathbb{R})$ be a matrix which is diagonalizable in $M_n(\mathbb{C})$. We note $u_{A, \mathbb{R}}: \mathbb{R}^n \rightarrow \mathbb{R}^n$ the endomorphism defined by the left ...
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0 votes
0 answers
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A decomposition of a vector space using a nilpotent linear map

Let $E$ be a vector space over a field $F$ and $u$ a linear map $E \rightarrow E$ which is nilpotent, ie, $u^p = 0$. I want to show that there exists a decomposition $E = F_1 \oplus ... \oplus F_p$. ...

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