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Thursday, 27 September 2007

nt.number theory - Diagonalizing matrices over cyclotomic fields with unitaries

Let F be a number field with a fixed embedding FhookrightarrowmathbbC such that the restriction of complex conjugation from mathbbC to F is in Gal(F/mathbbQ) and fix a Hermitian inner product langlev,wrangle=overlinev1w1+cdots+overlinevnwn on mathbbCn (with respect to the standard basis of mathbbCn. In particular, this restricts to a Hermitian inner product on Fn.



Now suppose we are given a unitary matrix U on Fn with respect to that inner product. It is well known (independent of unitarity) that U is diagonalizable over some extension E/F - for instance, take E to be a splitting field of the minimal polynomial of U. This means there is a matrix M over E such that MUM1 is diagonal in the standard basis.



What if we instead want a unitary W such that WUWdagger is diagonal? This can be accomplished by working over a bigger extension E/E that includes some extra square roots of elements of E. Namely, given any M that diagonalizes U over E, just add in the square roots of the eigenvalues of MdaggerM.



Now for my question:



Is there any sort of intrisic (i.e. independent of a choice of M) understanding of the extension E? By understanding, I mean things like: is there a nice way of describing its generators over E? Can anything be said about its Galois group in general? When is it a semidirect product?



My actual interest is in the case where F is cyclotomic and U has finite order (and thus has roots of unity as eigenvalues, so E is another cyclotomic field). Any advice on what is known in this specific, or otherwise the general case, would be be much appreciated.

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