Project IV (MATH4072) 2012-13


Quaternionic Linear Algebra (J R Parker)

Description

The quaternions were discovered on 16th October 1843 by Sir William Rowan Hamilton. He published his discovery in the Proceedings of the Royal Irish Academy less than a month later!

The quaternions are a generalisation of the complex numbers where we allow three distinct square roots of minus 1, called i, j, k. These anti-commute, meaning that ij=-ji=k, jk=-kj=i, ki=-ik=j. A quaternion is a+bi+cj+dk where a, b, c, d are real numbers.

We can form matrices of quaternions and then try to mimic constructions from linear algebra such as inverting a matrix, finding eigenvalues and eigenvectors, diagonalising a matrix. The fact that quaternions do not commute makes these constructions much harder.

For example, in order to find eigenvalues of a 2x2 quaternionic matrix one needs to find the roots of the characteristic polynomial. In other words one needs to solve a quadratic equation. Think about how you might go about doing this without ever using commutativity!

In this project you will first learn about the quaternions and then take some aspect of linear algebra and try to generalise it to linear algebra defined over the quaternions. Many of the papers in the subject were written over 50 years ago. You will need to read these papers and give an accessible account of what they contain.

Resources

Web based resources about quaternions:
  • A web page from euclideanspace.
  • The entry in Wikipedia.

    Early papers on quaternionic linear algebra:

  • JL Brenner, Matrices of quaternions, Pacific J Maths 1 (1951) 329-335.
  • HSM Coxeter, Quaternions and reflections, American Math Monthly 53 (1946) 49-62.
  • HC Lee, Eigenvalues and canonical forms of matrices with quaternion coefficients, Proceedings of the Royal Irish Academy Section A 15 (1949) 253-260.
  • I Niven Equations in quaternions American Math Monthly 48 (1948) 654-661.

    More recent papers on quaternionic linear algebra:

  • B Foreman, Conjugacy invariants of Sl(2,H), Linear Algebra Appl 381 (2004) 24-35.
  • L Huang and W So, On left eigenvalues of a quaternionic matrix, Linear Algebra Appl 323 (2001) 105-116.
  • L Huang and W So, Quadtatic formulas for quaternions, Applied Maths Letters 15 (2002) 533-540
  • F Zhang, Quaternions and matrices of quaternions, Linear Algebra Appl 251 (1997) 21-57.