# If $A$ is a symmetric invertible matrix, and $B$ is an antisymmetric matrix, then under what conditions is $A+B$ invertible?

Let $A$ be a symmetric invertible $n \times n$ matrix, and $B$ an antisymmetric $n \times n$ matrix. Under what conditions is $A+B$ an invertible matrix? In particular, if $A$ is positive definite, is $A+B$ invertible?

This isn't homework, I am just curious. Assume all matrices have entries in $\mathbb{R}$.

Edit to include context:

This question comes from a question that popped up in my research on string theory. One is interested in (pseudo)-Riemannian manifolds equipped with a two-form gauge field, modelling a background in which a closed string is moving. The metric, $g$, is a symmetric covariant 2-tensor, while the $b$-field is an antisymmetric covariant 2-tensor. The metric is non-degenerate and therefore invertible. Choosing local coordinates for the manifold, we can express the metric and $b$ field as $n \times n$ matrices, say $A$ and $B$, where $A$ is invertible. There is an operation on string backgrounds called T-duality which, in this simplified context, acts by inverting the matrix $E = A + B$, and so I am therefore interested in which scenarios this procedure works. I am mainly interested in the context where $A$ is real, invertible and positive definite (positive eigenvalues), corresponding to a Riemannian metric $g$, although I have tried to be a bit more general in the wording of the question.

Where to start: The main issue I have is that I don't really have any criteria for when the sum of two matrices is invertible. Certainly if the determinant is non-zero then I will be happy, but the determinant is not additive, so I don't know how to approach this. In two dimensions I can construct a counterexample whenever A has negative determinant, but the situations I really care about have $det(A)>0$. I would like to find a general criterion for when $A+B$ is invertible.

• $$\begin{pmatrix} 1 & 0 \\ 0 & -1 \end{pmatrix} + \begin{pmatrix} 0 & -1 \\ 1 & 0 \end{pmatrix} = \begin{pmatrix} 1 & -1 \\ 1& -1 \end{pmatrix}$$ May 3 '18 at 1:45
• What makes you believe this might be the case? May 3 '18 at 1:45
• Whether it's homework or not, you're expected to contribute some effort when you ask a homework-style question. May 3 '18 at 17:35
• Edited to include context and my approach. May 3 '18 at 23:17
• If you are interested in the conditions under which $A+B$ is invertible, perhaps you should ask that question, instead? You might edit the title and body of the question to make that point. May 3 '18 at 23:48

Pick $B$ any anti-symmetric matrix which is not nilpotent, and $\lambda \neq 0$ an eigenvalue of $B$.

Set $$A=-\lambda I$$

• perfect solution, $+1$. May 3 '18 at 1:46
• Seconded. +2 ${}$ May 3 '18 at 2:21

Nah. Take $$A = \begin{pmatrix} 1 & 0\\ 0 & -1 \end{pmatrix} \quad \mbox{and} \quad B = \begin{pmatrix} 0 & 1 \\ -1 & 0 \end{pmatrix}.$$Then $A$ is invertible and $$A+B = \begin{pmatrix} 1 & 1 \\ -1 & -1 \end{pmatrix}$$has determinant $\det(A+B) = 0$.

(Every matrix is the sum of a symmetric matrix and an anti-symmetric matrix. Take a non-invertible matrix, decompose it like that and see if the symmetric part is invertible.)

• This is precisely the context of where this question came from. I am interested in the situations in which the symmetric part ${is}$ invertible. In particular, I am looking for a solution which has $det(A) > 0$. May 3 '18 at 1:47
• If $A$ is invertible, then the matrix determinant lemma says that $$\det\left(\frac{A+A^\top}{2}\right) = \frac{\det A}{2^n} \det({\rm Id} + A^\top A^{-1}).$$ May 3 '18 at 1:52

If $A$ with positive determinant then in the case $n=2$, $A+B$ will be invertible. For $n\ge 3$, you can adjust the $2\times 2$ examples given in the other answers by adding a piece $(-1,1,\ldots)$ on the diagonal to make $A$ $n\times n$ with positive determinant.

Maybe you are thinking about the following: if $A$ is positive definite then the real parts of the eigenvalues of $A+B$ are positive ( and so $\ne 0$). This is easy to show but requires to look at complex vectors. A bit more general, considering complex matrices:

If $A_1$, $A_2$ are hermitian then the real parts of the eigenvalues of $A_1+ i A_2$ are between the the smallest and the largest eigenvalue of $A_1$, and the imaginary parts of the eingenvalues of $A_1 + i A_2$ are between the smallest and the largest eigenvalue of $A_2$.

Note that if $B$ real and skew symmetric then $1/i B$ is hermitian. Therefore, the eigenvalues of $B$ are purely imaginary and come in pairs, $i b$, $-i b$ (and perhaps some zero eigenvalues). So in the real case $B$ has "no input" towards invertibility of $A+B$. But a definite $A$ will guarantee invertibility of $A+B$.