The Unapologetic Mathematician

Mathematics for the interested outsider

The Jordan-Chevalley Decomposition

We recall that any linear endomorphism of a finite-dimensional vector space over an algebraically closed field can be put into Jordan normal form: we can find a basis such that its matrix is the sum of blocks that look like

\displaystyle\begin{pmatrix}\lambda&1&&&{0}\\&\lambda&1&&\\&&\ddots&\ddots&\\&&&\lambda&1\\{0}&&&&\lambda\end{pmatrix}

where \lambda is some eigenvalue of the transformation. We want a slightly more abstract version of this, and it hinges on the idea that matrices in Jordan normal form have an obvious diagonal part, and a bunch of entries just above the diagonal. This off-diagonal part is all in the upper-triangle, so it is nilpotent; the diagonalizable part we call “semisimple”. And what makes this particular decomposition special is that the two parts commute. Indeed, the block-diagonal form means we can carry out the multiplication block-by-block, and in each block one factor is a constant multiple of the identity, which clearly commutes with everything.

More generally, we will have the Jordan-Chevalley decomposition of an endomorphism: any x\in\mathrm{End}(V) can be written uniquely as the sum x=x_s+x_n, where x_s is semisimple — diagonalizable — and x_n is nilpotent, and where x_s and x_n commute with each other.

Further, we will find that there are polynomials p(T) and q(T) — each of which with no constant term — such that p(x)=x_s and q(x)=x_n. And thus we will find that any endomorphism that commutes with x with also commute with both x_s and x_n.

Finally, if A\subseteq B\subseteq V is any pair of subspaces such that x:B\to A then the same is true of both x_s and x_n.

We will prove these next time, but let’s see that this is actually true of the Jordan normal form. The first part we’ve covered.

For the second, set aside the assertion about p and q; any endomorphism commuting with x either multiplies each block by a constant or shuffles similar blocks, and both of these operations commute with both x_n and x_n.

For the last part, we may as well assume that B=V, since otherwise we can just restrict to x\vert_B\in\mathrm{End}(B). If \mathrm{Im}(x)\subseteq A then the Jordan normal form shows us that any complementary subspace to A must be spanned by blocks with eigenvalue 0. In particular, it can only touch the last row of any such block. But none of these rows are in the range of either the diagonal or off-diagonal portions of the matrix.

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August 28, 2012 - Posted by | Algebra, Linear Algebra

3 Comments »

  1. [...] now give the proof of the Jordan-Chevalley decomposition. We let have distinct eigenvalues with multiplicities , so the characteristic polynomial of [...]

    Pingback by The Jordan-Chevalley Decomposition (proof) « The Unapologetic Mathematician | August 28, 2012 | Reply

  2. [...] Now that we’ve given the proof, we want to mention a few uses of the Jordan-Chevalley decomposition. [...]

    Pingback by Uses of the Jordan-Chevalley Decomposition « The Unapologetic Mathematician | August 30, 2012 | Reply

  3. [...] first thing we do is take the Jordan-Chevalley decomposition of — — and fix a basis that diagonalizes with eigenvalues . We define to be the [...]

    Pingback by A Trace Criterion for Nilpotence « The Unapologetic Mathematician | August 31, 2012 | Reply


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