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What is the relationship between the definition for a matrix to be circulant and to be normal? Does one imply the other?

Assume matrix $A$ is symmetric, then $A^T=A$ and clearly it is normal, but not circulant in general. However, if I assume that $A$ is circulant, looks like $A^TA=AA^T$, so is it normal?

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    What's a circulant matrix?2012-07-11
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    @Rasmus: [a special kind of Toeplitz matrix](http://mathworld.wolfram.com/CirculantMatrix.html).2012-07-11
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    Look up the definition here: http://en.wikipedia.org/wiki/Circulant_matrix2012-07-11
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    [Very related](http://math.stackexchange.com/questions/142909) (in fact, I think my answer there also answers your question). Yes, circulant matrices are diagonalizable, and thus normal.2012-07-11
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    @J.M.: I know normal implies diagonalizable, now you're suggesting the converse is true as well? However, since all circulant matrices commute, it is obvious they are normal.2012-07-11
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    Oops, I forgot the adjective "unitarily" @Marc... to amend, "circulant matrices are *unitarily* diagonalizable (since the Fourier matrix is unitary), and they are thus normal".2012-07-11

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(as I just found out on Wikipedia) Normal matrices are those matrices that are diagonalisable with respect to some othonormal basis for the standard (Hermitian) inner product of $\mathbb C^n$. And circulant matrices are those that are diagonalisable with respect to one particular basis, formed of vectors $\zeta^0,\zeta^1,\ldots,\zeta^{n-1}$ where $\zeta$ is an $n$-th root of unity (and runs through all such roots as one runs through the basis). This basis is othonormal for the standard inner product, so "circulant matrix" is a very special case of "normal matrix".

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    "with respect to one particular basis" - yes, that's the Fourier matrix I was talking about, and mentioned in my answer to the related question.2012-07-11
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