Special conformal transformation

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In mathematical physics, a special conformal transformation is a type of spherical wave transformation and an expression of conformal symmetry.

Special conformal transformations arise from translation of spacetime and inversion. The inversion can be taken[1] to be multiplicative inversion of biquaternions B. The complex algebra B can be extended to P(B) through the projective line over a ring. Homographies on P(B) include translations:

U(q,1) \begin{pmatrix}1 & 0 \\ t & 1 \end{pmatrix} = U(q + t, 1).

The homography group G(B) includes

\begin{pmatrix}0 & 1 \\ 1 & 0 \end{pmatrix}\begin{pmatrix}1 & 0 \\ t & 1\end{pmatrix}\begin{pmatrix}0 & 1 \\ 1 & 0 \end{pmatrix} = \begin{pmatrix}1 & t \\ 0 & 1\end{pmatrix},

which provides the action of a special conformal transformation.

A coordinate grid prior to a special conformal transformation
The same grid after a special conformal transformation

Vector presentation[edit]

A special conformal transformation can also be written[2]

 x'^\mu = \frac{x^\mu-b^\mu x^2}{1-2b\cdot x+b^2x^2} \,.

It is a composition of an inversion (xμ → xμ/x2), a translation (xμ → xμ − bμ), and an inversion:

 \frac{x'^\mu}{x'^2} = \frac{x^\mu}{x^2} - b^\mu \,.

Its infinitesimal generator is

 K_\mu = -i(2x_\mu x^\nu\partial_\nu - x^2\partial_\mu) \,.


  1. ^ Arthur Conway (1911) "On the application of quaternions to some recent developments of electrical theory", Proceedings of the Royal Irish Academy 29:1–9, particularly page 9
  2. ^ Di Francesco; Mathieu, Sénéchal (1997). Conformal field theory. Graduate texts in contemporary physics. Springer. pp. 97–98. ISBN 978-0-387-94785-3.