What is Representations: Definition and 216 Discussions

Representations is an interdisciplinary journal in the humanities published quarterly by the University of California Press. The journal was established in 1983 and is the founding publication of the New Historicism movement of the 1980s. It covers topics including literary, historical, and cultural studies. The founding editorial board was chaired by Stephen Greenblatt and Svetlana Alpers. Representations frequently publishes thematic special issues, for example, the 2007 issue on the legacies of American Orientalism, the 2006 issue on cross-cultural mimesis, and the 2005 issue on political and intellectual redress.

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  1. B

    A Notation/Site for Representations of an Algebra

    I'm currently reading the paper "Higher Spin extension of cosmological spacetimes in 3d: asymptotically flat behaviour with chemical potentials in thermodynamics" I'm looking at equation (3) on page 4. I know that symmetrization brackets work like this A_(a b) = (A_ab + A_ba)/2. However I have...
  2. S

    I Understanding SU(2) Representations and Their Role in Particle Physics

    Hello! I just started reading about SU(2) (the book is Lie Algebras in Particle Physics by Howard Georgi) and I am confused about something - I attached a screenshot of those parts. So, for what I understood by now, the SU(2) are 2x2 matrices whose generators are Pauli matrices and they act on a...
  3. J

    I Unraveling Representations of SU2 & SU3 in Particle Physics

    Hi, I'm recently reading some text on particle physics and there is a section on symmetries and group theory. It gave the definition of SU2 as the group of unitary 2*2 matrices and that SU3 is the group of unitary 3*3 matrices. However, it kind of confuses me when it mentioned representations...
  4. J

    A Question about continued fraction representations

    How powerful are continued fraction representations? From what I understand, they could be used to exactly represent some irrational numbers So, could they represent any root of an nth degree polynomial equation? Specially where n>4, since 5th degree roots are not guaranteed to have an...
  5. hideelo

    Completeness and orthogonality of unitary irreducible representations

    I'm reading Lie Algebras and Particle Physics by Howard Georgi. He is trying to prove (section 1.12) that the matrix elements of the unitary irreducible representations (irreps) form a vector space of dimension N where N is the order of the group. For example for the matrix of the kth unitary...
  6. JonnyMaddox

    Explicitly Deriving Spinor Representations from Lorentz Group

    I'm currently reading a book on relativistic field theory and I'm trying to understand spinors. After the author introduces the four parts of the Lorentz group he talks about spinors and group representations: "...With this concept we see that the 2x2 unimodular matrices A discussed in the...
  7. D

    Representations of SU(3) Algebra

    Homework Statement I'm trying to figure out this question: "Show that the 10-dimensional representation R3,0 of A2 corresponds to a reducible representation of the LC[SU(2)] subalgebra corresponding to any root. Find the irreducible components of this representation. Does the answer depend on...
  8. Fredrik

    Insights Matrix Representations of Linear Transformations - Comments

    Fredrik submitted a new PF Insights post Matrix Representations of Linear Transformations Continue reading the Original PF Insights Post.
  9. Ravi Mohan

    Poincare group representations

    My question concerns both quantum theory and relativity. But since I came up with this while studying QFT from Weinberg, I post my question in this sub-forum. As I gather, we first work out the representation of Poincare group (say ##\mathscr{P}##) in ##\mathbb{R}^4## by demanding the Minkowski...
  10. Anchovy

    Standard Model decompositions of larger group representations?

    When reading about GUTs you often come across the 'Standard Model decomposition' of the representations of a given gauge group. ie. you get the Standard Model gauge quantum numbers arranged between some brackets. For example, here are a few SM decompositions of the SU(5) representations...
  11. Andre' Quanta

    Representations of Poincare group

    I need to study in detail the rappresentations of the Poincare Group, i am interessed in the idea that particles can be wieved as irriducible representations of it. Do you have some references about it?
  12. S

    A Problem about representations of group and particles

    I'm reading a paper these days, How can I get 2.28? It seems for a D-dim SO(2) gauge field, we have spin2, spin1, as well as spin0 particles?
  13. Safinaz

    Direct product of two representations

    Hi their, It's a group theory question .. it's known that ## 10 \otimes 5^* = 45 \oplus 5, ## Make the direct product by components: ##[ (1,1)^{ab}_{1} \oplus (3,2)^{ib}_{1/6} \oplus (3^*,1)^{ij}_{-2/3} ] \otimes [ (1,2)_{ c~-1/2} \oplus (3^*,1)_{ k~1/3} ] = (1,2)^{ab}_{ c~1/2} \oplus...
  14. S

    Meaning of representations of groups in different dimensions

    Problem This is a conceptual problem from my self-study. I'm trying to learn the basics of group theory but this business of representations is a problem. I want to know how to interpret representations of a group in different dimensions. Relevant Example Take SO(3) for example; it's the...
  15. Shackleford

    Find power series representations of the general solution

    Homework Statement (1+x2) y'' + 2xy' = 0 in powers of x Homework Equations y'' = \sum_{n=2}^{\infty} (n-1)na_nx^{n-2} y' = \sum_{n=1}^{\infty} na_nx^{n-1} The Attempt at a Solution (1+x2) y'' + 2xy' = (1+x^2) \sum_{n=2}^{\infty} (n-1)na_nx^{n-2} + 2x \sum_{n=1}^{\infty} na_nx^{n-1}...
  16. J

    Representations on Hilbert space

    Hello, I have some troubles understanding Hilbert representations for the standard free quantum particle On the one hand, we can represent Heisenberg algebra [Xi,Pj]= i delta ij on the space of square integrable functions on, say, R^3, with the X operator represented as multiplication and P...
  17. B

    Representations of semi-simple lie algebra

    Hello everybody, in Schwartz' QFT book it says (p. 483 - 484) In Problem 25.3 this is repeated asking the reader for a proof. I wonder though if this is really true. I know this can be proven for Lie algebras of compact Lie groups (or to be precise, every representation is equivalent to a...
  18. L

    Su(2), so(3) and their representations

    I try to understand the statement "Every representation of SO(3) is also a representation of SU(2)". Does that mean that all the matrices of an integer-spin rep of SU(2) are identical to the matrices of the corresponding spin rep of SO(3)? Say, the j=1 rep of SU(2) has three 3x3 matrices, so...
  19. JonnyMaddox

    Tensor product and representations

    Hi, I that <I|M|J>=M_{I}^{J} is just a way to define the elements of a matrix. But what is |I>M_{I}^{J}<J|=M ? I don't know how to calculate that because the normal multiplication for matrices don't seem to work. I'm reading a book where I think this is used to get a coordinate representation of...
  20. G

    Fundamental and contravariant representations

    The invariant of SL(2,C) is proven to be invariant under the action of the group by the following \epsilon'_{\alpha\beta} = N_{\alpha}^{\rho}N_{\beta}^{\sigma}\epsilon_{\rho\sigma}=\epsilon_{\alpha\beta}detN=\epsilon_{\alpha\beta} The existence of an invariant of this form (with two indices...
  21. S

    Components of adjoint representations

    In the way of defining the adjoint representation, \mathrm{ad}_XY=[X,Y], where X,Y are elements of a Lie algebra, how to determine the components of its representation, which equals to the structure constant?
  22. pellman

    Irreducible representations of SL(2,C)?

    The group of SL(2,C) is sometimes defined (by physicists) as the group of 2 X 2 complex matrices of determinant = 1. But then we can talk about other representations of SL(2,C). So apparently the set of 2 X 2 complex matrices of determinant = 1 is but one representation of SL(2,C). If so...
  23. ShayanJ

    Some questions about group representations

    I was rethinking about some things I learned but I came to things that seemed to be not firm enough in my mind. 1) When we want to find the unitary matrix that block-diagonalizes a certain matrix through a similarity transformation, we should find the eigenvectors of that matrix and stick them...
  24. G

    Spinor Representations: Intuitive Understanding

    Can you give me an intuitive understanding of the following: "The spin states of massive and massless Majorana spinors transform in representations of SO(D-1) and SO(D-2), respectively". I see the similarity with vectors bosons, where massive vectors have d-1 degrees of freedom and massless...
  25. S

    MHB Problem about a group with two inner direct product representations

    The problem: Suppose G is Abelian with two representations as the internal direct product of subgroups: G=HxK1, G=HxK2. Assume K1 is a subset of K2 and show K1=K2. My attempted solution: I took the element (e_H, k_2), where e_H is the identity element of H and k_2 is an arbitrary element in K2...
  26. K

    Help -- Writing Lagrangian in different Representations

    Okay, so I am trying to understand on how to write Lagrangian in different representations. I know the formula of the SU(3) lagrangian in terms of the 3 and 3* rep. Now presume I have a model in the SU(3) 10 plet rep which includes exotic fermions not in the SM. How would I write out the...
  27. Math Amateur

    MHB You're welcome, Peter! I'm glad it was helpful.

    I am reading "Introduction to Ring Theory" by P. M. Cohn (Springer Undergraduate Mathematics Series)[/COLOR] In Chapter 2: Linear Algebras and Artinian Rings, Cohn introduces representations of k-algebras as follows: So, essentially Cohn considers a right multiplication: \rho_a \ : \ x \mapsto...
  28. Math Amateur

    MHB Regular representations of finite dimensional algebras

    I am reading "Introduction to Ring Theory" by P. M. Cohn (Springer Undergraduate Mathematics Series) In Chapter 2: Linear Algebras and Artinian Rings we read the following on page 57: https://www.physicsforums.com/attachments/3149I am trying to gain an understanding of representations. I would...
  29. T

    Correct Representations of the Momentum Principle

    Homework Statement There are two parts to this question... a)Which of the following are correct representations of The Momentum Principle? (assuming a small-enough Δt whenever it shows up) 1) \frac{Δ\vec{p}}{Δt} = \vec{F}|| + \vec{F}⊥ 2) For every action there is an equal and opposite...
  30. PsychonautQQ

    Irreducible Representations

    Homework Statement Let V have dimension 3 and consider P_1(V ) = P(1,0,0) = span of {x,y,z}.Let I denote the subspace of all polynomials in P_1 of the form {rx+ry +rz|r any scalar}.Let W denote the subspace of all polynomials in P_1 of the form {rx+sy+tz|r+s+t = 0}. I and W are S_3 invariant...
  31. S

    Representations, states and tensors

    Hi. I am currently studying about representations of Lie algebras. I have two questions: 1. As I understand, when we say a "representation" in the context of Lie algebras, we don't mean the matrices (with the appropriate Lie algebra) but rather the states on which they act. But then, the...
  32. R

    Those Pictures (Representations) in QM and the density equations

    Hi there: I am reading a book (Atom-Photon interaction by Claude Cohen-Tannoudji, Page 448) and the following things gave a big headache. (1) Is there a density equation in Schrodinger Picture. because I encounter one, like: ##i \hbar \frac{d \sigma}{dt}=[\hat{H}, \sigma]## and...
  33. S

    Are Vectors Defined by Commutation Relations Always Roots in Any Representation?

    The vectors \vec{\alpha}=\{\alpha_1,\ldots\alpha_m \} are defined by [H_i,E_\alpha]=\alpha_i E_\alpha they are also known to be the non-zero weights, called the roots, in the adjoint representation. My question is - is this connection (that the vectors \vec{\alpha} defined by the commutation...
  34. E

    Projective representations of the spin group

    To define spinors in QM, we consider the projective representations of SO(n) that lift to linear representations of the double cover Spin(n). Why don't we consider projective representations of Spin?
  35. M

    Equality involving matrix exponentials / Lie group representations

    We have that A and B belong to different representations of the same Lie Group. The representations have the same dimension. X and Y are elements of the respective Lie algebra representations. A = e^{tX} B = e^{tY} We want to show, for a specific matrix M B^{-1} M B = AM Does it suffice to...
  36. C

    MHB Invariant subspaces of representations

    Let $\varrho :\mathbb{Z}\rightarrow GL_3(\mathbb{R})$ be the representation given by $\varrho (n)=A^n$ where A=$\begin{pmatrix} 2 & 5 & -1 \\ 2 & \frac{5}{2} & \frac{11}{2} \\ 6 & \frac{-2}{2} & \frac{3}{2} \\ \end{pmatrix}$ Does ρ have any 1-dimensional invariant subspaces? Do I have to...
  37. C

    Obtaining representations of the symmetric group

    Homework Statement Consider the following permutation representations of three elements in ##S_3##: $$\Gamma((1,2)) = \begin{pmatrix} 0&1&0\\1&0&0\\0&0&1 \end{pmatrix}\,\,\,\,;\Gamma((1,3)) = \begin{pmatrix} 0&0&1\\0&1&0\\1&0&0 \end{pmatrix}\,\,\,\,\,; \Gamma((1,3,2)) = \begin{pmatrix}...
  38. V

    Spin orbit and double group representations

    I am reading a text about the splitting of the energy levels in crystals caused by the spin orbit interaction. In particular, the argument is treated from the point of view of the group theory. The text starts saying that a representation (TxD) for the double group can be obtained from the...
  39. WannabeNewton

    Understanding Spinorial Representations in Relativistic QM/QFT

    Hey guys. I'm trying to understand spinorial representations in relativistic QM/QFT. If I make any mistakes in my statements (drawing mostly from chapter 2 of Maggiore) please correct them. In QM things are simple enough. We have a different representation of ##SU(2)## for each fixed...
  40. J

    Representations of the Fourier's integral

    If exist 3 representations for Fourier series (sine/cosine, exponential and amplite/phase) and at least two Fourier integral that I know f(t)=\int_{0}^{\infty }A(\omega)cos(\omega t) + B(\omega)sin(\omega t)d\omega f(t)=\int_{-\infty }^{+\infty }\frac{e^{+i\omega t}}{\sqrt{2\pi}}...
  41. J

    Representations of periodic functions

    Correct me if I'm wrong, but exist 3 forms for represent periodic functions, by sin/cos, by exp and by abs/arg. I know that given an expression like a cos(θ) + b sin(θ), I can to corvert it in A cos(θ - φ) or A sin(θ + ψ) through of the formulas: A² = a² + b² tan(φ) = b/a sin(φ) = b/A...
  42. K

    Regarding representations of the Lorentz group

    Hello! I'm currently reading Peskin and Schroeder and am curious about a qoute on page 38, which concerns representations of the Lorentz group. ”It can be shown that the most general nonlinear transformation laws can be built from these linear transformations, so there is no advantage in...
  43. W

    Spinor representations decomposed under subgroups in Joe's big book

    The question is about the spinor representation decomposed under subgroups. It's a common technique in string theory when parts of dimensions are compactified and ignored, and we are only interested in the remaining sub-symmetry. I'm learning it from the appendix B in Polchinski's big book...
  44. N

    Trying to interpret matrix representations of operators

    Say I have a 3x3 operator Q and I find its eigenvectors and eigenvalues. Now i know that those eigenvectors are the same as eigenfunctions so if i act on them with Q i will get the corresponding eigenvalue. What the question I am trying to solve asks is, Measure the quantity Q in state [b]...
  45. i_hbar

    Different representations of spinors

    First, greetings from newbie to "staff" Now, let's start: Since some days I'm struggling a little bit with this paper: http://jmp.aip.org/resource/1/jmapaq/v5/i9/p1204_s1?isAuthorized=no , especially with two questions: 1) On page 1205, II, A (right column): What does \tilde v B...
  46. T

    Matrix representations of angular momentum operators

    Homework Statement Write down the 3×3 matrices that represent the operators \hat{L}_x, \hat{L}_y, and \hat{L}_z of angular momentum for a value of \ell=1 in a basis which has \hat{L}_z diagonal. The Attempt at a Solution Okay, so my basis states \left\{\left|\ell,m\right\rangle\right\}...
  47. T

    Weinberg discussion on induced representations in his QFT Vol.1 Ch. 2

    This is discussed in Weinberg's Quantum Theory of Fields, in the chapter on Relativistic Quantum Mechanics. The point I am somewhat confused about occurs on page 63 - 64, if you have the book. He operates on a single particle state with the unitary homogeneous lorentz transformation...
  48. L

    Finding irreducible representations

    Hello! Can someone explain to me, as clearly as possible, how one can find irreducible representations of Lie groups (and especially in the context of finding the spectrum of e.g. the bosonic string theory)? I am following BB&S and Polchinski but I cannot really understand how they...
  49. D

    Useful Representations of Log Audio Frequency Spectrum

    Hi everyone, So I'm trying to basically generate a list of numbers between 20 and 20,000 (Hz) in log space that will give good resolution to parts of the audio spectrum that matter! After all that is the point of using log scale for frequency in the first place. The list generator I have...
  50. S

    Transforming Representations of SO(3) to Act on Vectors?

    Hi Everybody! I am working on QFT and learning representation theory from Coleman's lecture notes. Just the necessary stuff to go to the Dirac equation. To my question: From the generators of SO(3) I get through exponentiation an element of SO(3), this holds naturally for any Lie group...
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