## A Matrix Equation of a Symmetric Matrix and the Limit of its Solution

## Problem 457

Let $A$ be a real symmetric $n\times n$ matrix with $0$ as a simple eigenvalue (that is, the algebraic multiplicity of the eigenvalue $0$ is $1$), and let us fix a vector $\mathbf{v}\in \R^n$.

**(a)** Prove that for sufficiently small positive real $\epsilon$, the equation

\[A\mathbf{x}+\epsilon\mathbf{x}=\mathbf{v}\]
has a unique solution $\mathbf{x}=\mathbf{x}(\epsilon) \in \R^n$.

**(b)** Evaluate

\[\lim_{\epsilon \to 0^+} \epsilon \mathbf{x}(\epsilon)\]
in terms of $\mathbf{v}$, the eigenvectors of $A$, and the inner product $\langle\, ,\,\rangle$ on $\R^n$.

(*University of California, Berkeley, Linear Algebra Qualifying Exam*)