Algebra

Info icon This preview shows pages 1–2. Sign up to view the full content.

Math 150a: Modern Algebra Homework 9 Solutions GK1. For which n is the dihedral group D n equiconjugate with O ( 2 ) ? Solution: Let x , y D n be conjugate in O ( 2 ) . Then both x and y are rotations or reflections. If x and y are both rotations, then they must both rotate by the same angle (up to sign). Then either x = y , or x = y 1 . The solution to GK2 on homework #5 covers the conjugacy classes of D n . And, in either case, x and y are conjugate in D n . If x and y are both reflections, then they might not be conjugate in D n if n is even. But they have to be conjugate in D n when n is odd. Hence D n is equiconjugate with O ( 2 ) exactly when n is odd. square GK2. In class I discussed Theorem 5.2.2 from the book, which says that every element of Isom ( R 2 ) is either the identity, a rotation, a reflection, a translation, or a glide reflection. a. Express each of these types of isometries as a product of at most three reflections. Solution: Let φ be a rotation about a point p R 2 by an angle θ . Now let l 1 and l 2 be two lines through p , so that the angle between l 1 and l 2 (measured counterclockwise from l 1 to l 2 ) is equal to θ / 2. Then, if f 1 and f 2 are the reflections through l 1 and l 2 (respectively); then φ = f 2 f 1 . If φ is a reflection of R 2 , it is clearly the product of at most three reflections. θ p θ / 2 l 1 l 2 f 1 f 2 (a rotation by angle θ ) (a translation of distance t ) t −→ a t / 2 f 1 f 2 −→ a t / 2 (a glide reflection) l Next, let φ be the translation of R 2 p mapsto→ p +
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

Image of page 2
This is the end of the preview. Sign up to access the rest of the document.
  • Spring '03
  • Kuperberg
  • Algebra

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern