Haynsworth inertia additivity formula
# Haynsworth inertia additivity formula

In mathematics, the Haynsworth inertia additivity formula, discovered by Emilie Virginia Haynsworth (1916–1985), concerns the number of positive, negative, and zero eigenvalues of a Hermitian matrix and of block matrices into which it is partitioned.

The inertia of a Hermitian matrix H is defined as the ordered triple

    $\mathrm {In} (H)=\left(\pi (H),\nu (H),\delta (H)\right)$

whose components are respectively the numbers of positive, negative, and zero eigenvalues of H. Haynsworth considered a partitioned Hermitian matrix

    $H={\begin{bmatrix}H_{11}&H_{12}\\H_{12}^{\ast }&H_{22}\end{bmatrix}}$

where $H_{11}$ is nonsingular and $H_{12}^*$ is the conjugate transpose of $H_{12}.$ The formula states:

    $\mathrm {In} {\begin{bmatrix}H_{11}&H_{12}\\H_{12}^{\ast }&H_{22}\end{bmatrix}}=\mathrm {In} (H_{11})+\mathrm {In} (H/H_{11})$

where $H/H_{11}$ is the Schur complement of $H_{11}$ in H:

    $H/H_{11}=H_{22}-H_{12}^{\ast }H_{11}^{-1}H_{12}.$

## Generalization

If $H_{11}$ is singular, we can still define the generalized Schur complement, using the Moore–Penrose inverse $H_{11}^{+}$ instead of $H_{11}^{-1}$.

The formula does not hold if $H_{11}$ is singular. However, a generalization has been proven in 1974 by Carlson, Haynsworth and Markham, to the effect that $\pi (H)\geq \pi (H_{11})+\pi (H/H_{11})$ and $\nu (H)\geq \nu (H_{11})+\nu (H/H_{11})$.

Carlson, Haynsworth and Markham also gave sufficient and necessary conditions for equality to hold.

