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Applicable only to sparse matrices SpMat. Applicable to Mat only. Saving int , float or double matrices is a lossy operation, as each element is copied and converted to an 8 bit representation. As such the matrix should have values in the [0,] interval, otherwise the resulting image may not display correctly. Applicable to Cube only.

Numerical data stored in portable HDF5 binary format. Mahalanobis distance, which uses a global diagonal covariance matrix estimated from the training samples; this is recommended for probabilistic applications. Newtonian constant of gravitation in newton square meters per kilogram squared. Planck constant over 2 pi, aka reduced Planck constant in joule seconds. Disable going through the run-time Armadillo wrapper library libarmadillo. GR , math. RT Tags: CA groups , characters , classification of finite simple groups , Fourier transform , Frobenius groups , Frobenius theorem , induced representations , integrality gap , Suzuki theorem by Terence Tao 18 comments.

The classification of finite simple groups CFSG , first announced in but only fully completed in , is one of the monumental achievements of twentieth century mathematics. An important precursor of the CFSG is the Feit-Thompson theorem from , which asserts that every finite group of odd order is solvable, or equivalently that every non-abelian finite simple group has even order. This is an immediate consequence of CFSG, and conversely the Feit-Thompson theorem is an essential starting point in the proof of the classification, since it allows one to reduce matters to groups of even order for which key additional tools such as the Brauer-Fowler theorem become available.

The original proof of the Feit-Thompson theorem is pages long, which is significantly shorter than the proof of the CFSG, but still far from short. While parts of the proof of the Feit-Thompson theorem have been simplified and it has recently been converted , after six years of effort, into an argument that has been verified by the proof assistant Coq , the available proofs of this theorem are still extremely lengthy by any reasonable standard.

However, there is a significantly simpler special case of the Feit-Thompson theorem that was established previously by Suzuki in , which was influential in the proof of the more general Feit-Thompson theorem and thus indirectly to the proof of CFSG.


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Define a CA-group to be a group with the property that the centraliser of any non-identity element is abelian; equivalently, the commuting relation defined as the relation that holds when commutes with , thus is an equivalence relation on the non-identity elements of. Trivially, every abelian group is CA. A non-abelian example of a CA-group is the group of invertible affine transformations on a field. A little less obviously, the special linear group over a finite field is a CA-group when is a power of two.

In view of the CFSG, we thus see that CA or nearly CA groups form an important subclass of the simple groups, and it is thus of interest to study them separately. To this end, we have. Moving even further down the ladder of simple precursors of CSFG is the following theorem of Frobenius from Define a Frobenius group to be a finite group which has a subgroup called the Frobenius complement with the property that all the non-trivial conjugates of for , intersect only at the origin.

For instance the group is also a Frobenius group take to be the affine transformations that fix a specified point , e. This example suggests that there is some overlap between the notions of a Frobenius group and a CA group. Indeed, note that if is a CA-group and is a maximal abelian subgroup of , then any conjugate of that is not identical to will intersect only at the origin because and each of its conjugates consist of equivalence classes under the commuting relation , together with the identity.

So if a maximal abelian subgroup of a CA-group is its own normaliser thus is equal to , then the group is a Frobenius group.

Then there exists a normal subgroup of called the Frobenius kernel of such that is the semi-direct product of and. Note that if every CA-group of odd order was either Frobenius or abelian, then Theorem 2 would imply Theorem 1 by an induction on the order of , since any subgroup of a CA-group is clearly again a CA-group. Then the set of permutations in that fix no points in , together with the identity, is closed under composition.

Again, a good example to keep in mind for this theorem is when is the group of affine permutations on a field i. In that case, the set of permutations in that do not fix any points are the non-trivial translations. To deduce Theorem 3 from Theorem 2 , one applies Theorem 2 to the stabiliser of a single point in.

Conversely, to deduce Theorem 2 from Theorem 3 , set to be the space of left-cosets of , with the obvious left -action; one easily verifies that this action is faithful, transitive, and each non-identity element of fixes at most one left-coset of basically because it lies in at most one conjugate of. If we let be the elements of that do not fix any point in , plus the identity, then by Theorem 3 is closed under composition; it is also clearly closed under inverse and conjugation, and is hence a normal subgroup of.

From construction is the identity plus the complement of all the conjugates of , which are all disjoint except at the identity, so by counting elements we see that. As normalises and is disjoint from , we thus see that is all of , giving Theorem 2.

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Despite the appealingly concrete and elementary form of Theorem 3 , the only known proofs of that theorem or equivalently, Theorem 2 in its full generality proceed via the machinery of group characters which one can think of as a version of Fourier analysis for nonabelian groups. The proofs of Feit-Thompson and CFSG also involve characters, but those proofs also contain many other arguments of much greater complexity than the character-based portions of the proof. But even for the simplest two results on this ladder — Frobenius and Suzuki — it seems remarkably difficult to find any proof that is not essentially the character-based proof.

In any case, I am recording here the standard character-based proofs of the theorems of Frobenius and Suzuki below the fold. There is nothing particularly novel here, but I wanted to collect all the relevant material in one place, largely for my own benefit. Let be Hermitian matrices, with eigenvalues and. The Harish-Chandra — Itzykson-Zuber integral formula exactly computes the integral.

There are at least two standard ways to prove this formula in the literature. One way is by applying the Duistermaat-Heckman theorem to the pushforward of Liouville measure on the coadjoint orbit or more precisely, a rotation of such an orbit by under the moment map , and then using a stationary phase expansion.

Another way, which I only learned about recently, is to use the formulae for evolution of eigenvalues under Dyson Brownian motion as well as the closely related formulae for the GUE ensemble , which were derived in this previous blog post. Both of these approaches can be found in several places in the literature the former being observed in the original paper of Duistermaat and Heckman , and the latter observed in the paper of Itzykson and Zuber as well as in this later paper of Johansson , but I thought I would record both of these here for my own benefit.

At first glance, this might suggest that these formulae could be of use in the study of the GOE ensemble, but unfortunately the Lie algebra associated to corresponds to real anti-symmetric matrices rather than real symmetric matrices. This also occurs in the case, but there one can simply multiply by to rotate a complex skew-Hermitian matrix into a complex Hermitian matrix.

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This is consistent, though, with the fact that the somewhat rarely studied anti-symmetric GOE ensemble has cleaner formulae in particular, having a determinantal structure similar to GUE than the much more commonly studied symmetric GOE ensemble. Let be a compact group. Throughout this post, all topological groups are assumed to be Hausdorff.

Then has a number of unitary representations , i. In particular, one has the left-regular representation , where we equip with its normalised Haar measure and the Borel -algebra to form the Hilbert space , and is the translation operation. We call two unitary representations and isomorphic if one has for some unitary transformation , in which case we write. Given two unitary representations and , one can form their direct sum in the obvious manner:. Conversely, if a unitary representation has a closed invariant subspace of thus for all , then the orthogonal complement is also invariant, leading to a decomposition of into the subrepresentations ,.

Accordingly, we will call a unitary representation irreducible if is nontrivial i. By the principle of infinite descent, every finite-dimensional unitary representation is then expressible perhaps non-uniquely as the direct sum of irreducible representations. The Peter-Weyl theorem asserts, among other things, that the same claim is true for the regular representation:. Theorem 1 Peter-Weyl theorem Let be a compact group.

Then the regular representation is isomorphic to the direct sum of irreducible representations. In fact, one has , where is an enumeration of the irreducible finite-dimensional unitary representations of up to isomorphism. It is not difficult to see that such an enumeration exists. In the case when is abelian, the Peter-Weyl theorem is a consequence of the Plancherel theorem ; in that case, the irreducible representations are all one dimensional, and are thus indexed by the space of characters i.