Dr Woit summarises this most obviously in Not really Incorrect (United kingdom ed., p47): ‘A Sit classification . contains enormous quantities out of facets continuously connected together with her. It actually was the newest signal idea of them teams you to Weyl was 
reading.
‘A simple instance of a rest classification including an expression is the fact of the group of rotations of these two-dimensional jet. This really is a proportion of planes. The point that are invariant is the distance between a spot with the flat plus the central section. This is the exact same pre and post the latest rotation. You can in reality determine rotations of your own plane while the accurately those changes you to get-off invariant the length with the main area. There’s a keen infinity of these transformations, however they can all be parametrised by the just one number, the fresh direction from rotation.
Argand diagram demonstrating rotation by a position to your cutting-edge jet. Illustration borrowing from the bank: based on Fig. 3.one in Not really Incorrect.
‘If pops into their heads the latest plane as advanced plane (the latest plane whoever one or two coordinates identity the real and you will fictional region out-of a complex number), then the rotations should be thought of as relevant not just in order to angles, but so you can a complicated number of length you to definitely. If a person multiplies every affairs about complex planes by a great considering complex amount of unit size, one has the involved rotation (this can be an easy exercise inside the influencing state-of-the-art numbers). Thus, the group from rotations from the advanced flat might be called the fresh new ‘unitary gang of transformations of one state-of-the-art variable’, and you may written You(1).
‘This is a highly specific icon of one’s category You(1), the fresh symbol since transformations of your own cutting-edge airplanes . something you should mention is that the transformation of rotation by the an angle try formally much like the sales away from a revolution of the altering their stage [of the Fourier investigation, and this means a great waveform away from wave amplitude rather than date just like the a beneficial frequency range graph showing wave amplitude in place of trend volume by the decomposing the original waveform to the a sequence which is the amount of an abundance of nothing sine and you may cosine trend efforts]. This series off changes of one’s phase of a revolution try similar to the succession out of rotations away from a plane in general increases the angle regarding rotation of 0 so you can 360 levels. Therefore example, U(1) symmetry changes usually are titled phase changes. .
Considering a two-dimensional airplane that have chosen main section, it’s possible to consider rotating brand new airplanes by the a given position in the brand new main point
‘In general, if a person features a random count Letter of cutting-edge number, you can establish the team from unitary transformations off Letter advanced variables and signify it U(N). As it happens that it’s a smart idea to break such changes towards the two parts: the brand new part that just multiplies the Letter state-of-the-art numbers by the same equipment advanced count (this region is actually good You(1) like just before), while the other people. Another region is the perfect place the difficulty was, and is also given the label regarding unique unitary changes off N (complex) details and indicates SU(N). Element of Weyl’s conclusion consisted in the a whole knowledge of the representations away from SU(N), for N, no matter how higher.
Considering a primary wave, if a person imagines copying they after which deciding to make the content so much more and a lot more of stage on initial revolution, eventually you to gets to in which one come, in phase to your first revolution
‘In the outcome Letter = step 1, SU(1) is only the trivial group having one to ability. The initial low-trivial situation is that away from SU(2) . carefully connected with the group off rotations inside around three real dimensions . the team from special orthagonal changes regarding about three (real) details . category So(3). The specific relation ranging from Very(3) and SU(2) is that each rotation in three dimensions represents a couple distinct elements of SU(2), otherwise SU(2) is actually particular sense a twofold variety of Very(3).’
