# Schur–Horn theorem

In mathematics, particularly linear algebra, the **Schur–Horn theorem**, named after Issai Schur and Alfred Horn, characterizes the diagonal of a Hermitian matrix with given eigenvalues. It has inspired investigations and substantial generalizations in the setting of symplectic geometry. A few important generalizations are Kostant's convexity theorem, Atiyah–Guillemin–Sternberg convexity theorem, Kirwan convexity theorem.

## Statement

**Theorem.** Let
and
be vectors in
such that their entries are in non-increasing order. There is a Hermitian matrix with diagonal values
and eigenvalues
if and only if

and

## Polyhedral geometry perspective

### Permutation polytope generated by a vector

The **permutation polytope** generated by
denoted by
is defined as the convex hull of the set
. Here
denotes the symmetric group on
. The following lemma characterizes the permutation polytope of a vector in
.

**Lemma**.[1][2] If
, and
then the following are equivalent :

(i) .

(ii)

(iii) There are points in such that and for each in , some transposition in , and some in , depending on .

### Reformulation of Schur–Horn theorem

In view of the equivalence of (i) and (ii) in the lemma mentioned above, one may reformulate the theorem in the following manner.

**Theorem.** Let
and
be real vectors. There is a Hermitian matrix with diagonal entries
and eigenvalues
if and only if the vector
is in the permutation polytope generated by
.

Note that in this formulation, one does not need to impose any ordering on the entries of the vectors and .

## Proof of the Schur–Horn theorem

Let be a Hermitian matrix with eigenvalues , counted with multiplicity. Denote the diagonal of by , thought of as a vector in , and the vector by . Let be the diagonal matrix having on its diagonal.

( ) may be written in the form , where is a unitary matrix. Then

Let be the matrix defined by . Since is a unitary matrix, is a doubly stochastic matrix and we have . By the Birkhoff–von Neumann theorem, can be written as a convex combination of permutation matrices. Thus is in the permutation polytope generated by . This proves Schur's theorem.

( ) If occurs as the diagonal of a Hermitian matrix with eigenvalues , then also occurs as the diagonal of some Hermitian matrix with the same set of eigenvalues, for any transposition in . One may prove that in the following manner.

Let be a complex number of modulus such that and be a unitary matrix with in the and entries, respectively, at the and entries, respectively, at all diagonal entries other than and , and at all other entries. Then has at the entry, at the entry, and at the entry where . Let be the transposition of that interchanges and .

Then the diagonal of is .

is a Hermitian matrix with eigenvalues . Using the equivalence of (i) and (iii) in the lemma mentioned above, we see that any vector in the permutation polytope generated by , occurs as the diagonal of a Hermitian matrix with the prescribed eigenvalues. This proves Horn's theorem.

## Symplectic geometry perspective

The Schur–Horn theorem may be viewed as a corollary of the Atiyah–Guillemin–Sternberg convexity theorem in the following manner. Let denote the group of unitary matrices. Its Lie algebra, denoted by , is the set of skew-Hermitian matrices. One may identify the dual space with the set of Hermitian matrices via the linear isomorphism defined by for . The unitary group acts on by conjugation and acts on by the coadjoint action. Under these actions, is an -equivariant map i.e. for every the following diagram commutes,

Let and denote the diagonal matrix with entries given by . Let denote the orbit of under the -action i.e. conjugation. Under the -equivariant isomorphism , the symplectic structure on the corresponding coadjoint orbit may be brought onto . Thus is a Hamiltonian -manifold.

Let denote the Cartan subgroup of which consists of diagonal complex matrices with diagonal entries of modulus . The Lie algebra of consists of diagonal skew-Hermitian matrices and the dual space consists of diagonal Hermitian matrices, under the isomorphism . In other words, consists of diagonal matrices with purely imaginary entries and consists of diagonal matrices with real entries. The inclusion map induces a map , which projects a matrix to the diagonal matrix with the same diagonal entries as . The set is a Hamiltonian -manifold, and the restriction of to this set is a moment map for this action.

By the Atiyah–Guillemin–Sternberg theorem, is a convex polytope. A matrix is fixed under conjugation by every element of if and only if is diagonal. The only diagonal matrices in are the ones with diagonal entries in some order. Thus, these matrices generate the convex polytope . This is exactly the statement of the Schur–Horn theorem.

## Notes

- Kadison, R. V., Lemma 5,
*The Pythagorean Theorem: I. The finite case*, Proc. Natl. Acad. Sci. USA, vol. 99 no. 7 (2002):4178–4184 (electronic) - Kadison, R. V.; Pedersen, G. K., Lemma 13,
*Means and Convex Combinations of Unitary Operators*, Math. Scand. 57 (1985),249–266

## References

- Schur, Issai,
*Über eine Klasse von Mittelbildungen mit Anwendungen auf die Determinantentheorie*, Sitzungsber. Berl. Math. Ges. 22 (1923), 9–20. - Horn, Alfred,
*Doubly stochastic matrices and the diagonal of a rotation matrix,*American Journal of Mathematics 76 (1954), 620–630. - Kadison, R. V.; Pedersen, G. K.,
*Means and Convex Combinations of Unitary Operators*, Math. Scand. 57 (1985),249–266. - Kadison, R. V.,
*The Pythagorean Theorem: I. The finite case*, Proc. Natl. Acad. Sci. USA, vol. 99 no. 7 (2002):4178–4184 (electronic)

## External links

- MathWorld
- Terry Tao:
*254A, Notes 3a: Eigenvalues and sums of Hermitian matrices* - Sheela Devadas, Peter J. Haine, Keaton Stubis:
*The Schur-Horn Theorem*