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Why does Transpose[m, {1, 1}] give the diagonal of m?


In a recent article in the thread Re: [mg1741] RealDigits, Roman Maeder explains
one way to test whether a matrix is diagonal:

>even faster and certainly simpler:
>dQ[m_?MatrixQ] /; SameQ @@ Dimensions[m] :=
>        m == DiagonalMatrix[Transpose[m, {1, 1}]]
>(note: Transpose[m, {1, 1}] returns the list of diagonal elements.)

An elegant test.  My question: why does Transpose[m, {1, 1}] return 
the list of diagonal elements?  Looking under Transpose in Wolfram's 
Mathematica Reference Guide (an appendix to Mathematica, 2nd ed.), 
I find:

>Transpose[list, {n1,n2, ...}] transposes list so that the kth level
>in list is the nkth level in the result.

I expected Depth[Transpose[list, {n1,n2, ...}]] to be the same as 
Depth[list].  I expected Transpose to swap levels; I did not
expect it to throw any away.  Evidently I am laboring under some
misconception.  Can someone explain what is going on?

-- 
David Cabana   drc at gate.net


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