# Inverse of the sum of matrices

I have two square matrices - $A$ and $B$. $A^{-1}$ is known and I want to calculate $(A+B)^{-1}$. Are there theorems that help with calculating the inverse of the sum of matrices? In general case $B^{-1}$ is not known, but if it is necessary then it can be assumed that $B^{-1}$ is also known.

• @Arturo: I know that they might not be invertible, but let's assume they are. @Adrian: Unfortunately I don't have direct access to jstor. – Tomek Tarczynski Jan 16 '11 at 20:58
• – Rajesh Dachiraju Oct 30 '18 at 11:18

In general, $A+B$ need not be invertible, even when $A$ and $B$ are. But one might ask whether you can have a formula under the additional assumption that $A+B$ is invertible.

As noted by Adrián Barquero, there is a paper by Ken Miller published in the Mathematics Magazine in 1981 that addresses this.

He proves the following:

Lemma. If $A$ and $A+B$ are invertible, and $B$ has rank $1$, then let $g=\mathrm{trace}(BA^{-1})$. Then $g\neq -1$ and $$(A+B)^{-1} = A^{-1} - \frac{1}{1+g}A^{-1}BA^{-1}.$$

From this lemma, we can take a general $A+B$ that is invertible and write it as $A+B = A + B_1+B_2+\cdots+B_r$, where $B_i$ each have rank $1$ and such that each $A+B_1+\cdots+B_k$ is invertible (such a decomposition always exists if $A+B$ is invertible and $\mathrm{rank}(B)=r$). Then you get:

Theorem. Let $A$ and $A+B$ be nonsingular matrices, and let $B$ have rank $r\gt 0$. Let $B=B_1+\cdots+B_r$, where each $B_i$ has rank $1$, and each $C_{k+1} = A+B_1+\cdots+B_k$ is nonsingular. Setting $C_1 = A$, then $$C_{k+1}^{-1} = C_{k}^{-1} - g_kC_k^{-1}B_kC_k^{-1}$$ where $g_k = \frac{1}{1 + \mathrm{trace}(C_k^{-1}B_k)}$. In particular, $$(A+B)^{-1} = C_r^{-1} - g_rC_r^{-1}B_rC_r^{-1}.$$

(If the rank of $B$ is $0$, then $B=0$, so $(A+B)^{-1}=A^{-1}$).

• Thanks, I was looking for something like this. – Tomek Tarczynski Jan 17 '11 at 15:18
• The lemma is the Sherman-Morrison formula, isn't it? – Rahul Apr 30 '14 at 8:15
• Can this theorem be used in finding the inverse of $${\large[}g_{\mu\nu}+\chi \frac{k_\mu k_\nu}{k^2}{\large]}$$ where $g$ is the Minkowski metric tensor and the $k$'s are four-vectors? Please see this question in Physics.SE: physics.stackexchange.com/q/141613/31965 Thanks. – Your Majesty Oct 16 '14 at 15:59
• What about the case of $\left( A + \lambda I \right)^{-1}$? Let's assume $A$ is PSD. – Royi Aug 22 '17 at 7:44
• I am also interested in the case $(\mathbf{A}+\mathbf{I})^{-1}$. Please see math.stackexchange.com/questions/2680914/… – TheDon Mar 9 '18 at 16:00

It is shown in On Deriving the Inverse of a Sum of Matrices that

$$(A+B)^{-1}=A^{-1}-A^{-1}B(A+B)^{-1}$$.

This equation cannot be used to calculate $$(A+B)^{-1}$$, but it is useful for perturbation analysis where $$B$$ is a perturbation of $$A$$. There are several other variations of the above form (see equations (22)-(26) in this paper).

This result is good because it only requires $$A$$ and $$A+B$$ to be nonsingular. As a comparison, the SMW identity or Ken Miller's paper (as mentioned in the other answers) requires some nonsingualrity or rank conditions of $$B$$.

This I found accidentally.

Suppose given $A$, and $B$, where $A$ and $A+B$ are invertible. Now we want to know the expression of $(A+B)^{-1}$ without imposing the all inverse. Now we follow the intuition like this. Suppose that we can express $(A+B)^{-1} = A^{-1} + X$, next we will present simple straight forward method to compute $X$ $$(A+B)^{-1} = A^{-1} + X$$ $$(A^{-1} + X) (A + B) = I$$ $$A^{-1} A + X A + A^{-1} B + X B = I$$ $$X(A + B) = - A^{-1} B$$ $$X = - A^{-1} B ( A + B)^{-1}$$ $$X = - A^{-1} B (A^{-1} + X)$$ $$(I + A^{-1}B) X = - A^{-1} B A^{-1}$$ $$X = - (I + A^{-1}B)^{-1} A^{-1} B A^{-1}$$

This lemma is simplification of lemma presented by Ken Miller, 1981

• Where did you find this? Can you give a citation? – Daniel Renshaw Jun 11 '13 at 10:55
• How is this a simplification of the lemma shown in Ken Miller 1981? Are we talking about "On the Inverse of the Sum of Matrices" or any other work? (In any case, I find this property quite useful, just need to cite it properly). – Rufo Apr 10 '14 at 15:15
• Interesting to notice that line 3 is a Sylvester equation. – ati Dec 11 '14 at 16:26
• In order to conclude last line,we must have (I+A^-1B) invertible. So how are we sure about that, It might be easy but (I am not getting. Can you please explain @ Muhammad Fuday. – Sry Feb 16 '15 at 5:43
• What about the case of $\left( A + \lambda I \right)^{-1}$? Let's assume $A$ is PSD. – Royi Aug 22 '17 at 7:45

$(A+B)^{-1} = A^{-1} - A^{-1}BA^{-1} + A^{-1}BA^{-1}BA^{-1} - A^{-1}BA^{-1}BA^{-1}BA^{-1} + \cdots$

provided $\|A^{-1}B\|<1$ or $\|BA^{-1}\| < 1$ (here $\|\cdot\|$ means norm). This is just the Taylor expansion of the inversion function together with basic information on convergence.

(posted essentially at the same time as mjqxxx)

A formal power series expansion is possible: $$\begin{eqnarray} (A + \epsilon B)^{-1} &=& \left(A \left(I + \epsilon A^{-1}B\right)\right)^{-1} \\ &=& \left(I + \epsilon A^{-1}B\right)^{-1} A^{-1} \\ &=& \left(I - \epsilon A^{-1}B + \epsilon^2 A^{-1}BA^{-1}B - ...\right) A^{-1} \\ &=& A^{-1} - \epsilon A^{-1} B A^{-1} + \epsilon^2 A^{-1} B A^{-1} B A^{-1} - ... \end{eqnarray}$$ Under appropriate conditions on the eigenvalues of $A$ and $B$ (such that $A$ is sufficiently "large" compared to $B$), this will converge to the correct result at $\epsilon=1$.

• The point about eigenvalues is apt, because this works even if $\|A^{-1}B\|\geq1$ and $\|BA{^-1}\|\geq1$ as long as the spectral radius of $A^{-1}B$ or $BA^{-1}$ is less than $1$. – Jonas Meyer Jan 17 '11 at 2:20
• What about the case of $\left( A + \lambda I \right)^{-1}$? Let's assume $A$ is PSD. – Royi Aug 22 '17 at 7:45

Assuming everything is nicely invertible, you are probably looking for the SMW identity (which, i think, can also be generalized to pseudoinverses if needed)

Please see caveat in the comments below; in general if $B$ is low-rank, then you'd be happy using SMW.

• It also requires $(A^{-1} + B^{-1})^{-1}$ to be known, doesn't it? – mjqxxxx Jan 16 '11 at 21:06
• The Sherman-Morrison "update" formula is most efficient if $B$ is of low rank. So the usual application (rank one or two if symmetry is to be preserved) doesn't require $B^{-1}$ to exist. – hardmath Jan 16 '11 at 21:07
• @mjqxxxx: yes, actually smw does require that inverse, which actually renders this answer useless, unless one is looking for inverses where $B$ is low-rank, and is written as $B=UCV^T$. – user1709 Jan 16 '11 at 22:01

I'm surprising that no one realize it's a special case of the well-known matrix inverse lemma or [Woodbury matrix identity], it says,

$\left(A+UCV \right)^{-1} = A^{-1} - A^{-1}U \left(C^{-1}+VA^{-1}U \right)^{-1} VA^{-1}$ ,

just set U=V=I, it immediately gets

$\left(A+C \right)^{-1} = A^{-1} - A^{-1} \left(C^{-1}+A^{-1} \right)^{-1} A^{-1}$ .

It is possible to come up with pretty simple examples where $A$,$A^{-1}$,$B$, and $B^{-1}$ are all very nice, but applying $(A+B)^{-1}$ is considered very difficult.

The canonical example is where $A = \Delta$ is a finite difference implementation of the Laplacian on a regular grid (with, for example, Dirichlet boundary conditions), and $B=k^2I$ is a multiple of the identity. The finite difference laplacian and it's inverse are very nice and easy to deal with, as is the identity matrix. However, the combination $$\Delta + k^2 I$$ is the Helmholtz operator, which is widely known as being extremely difficult to solve for large $k$.

If A and B were numbers, there is no simpler way to write $\frac{1}{A+B}$ in term of $\frac{1}{A}$ and $B$ so I don't know why you would expect there to be for matrices. It is even possible to have matrices, A and B, so that neither $A^{-1}$ and $B^{-1}$ exist but $(A+B)^{-1}$ does or, conversely, such that both $A^{-1}$ nor $B^{-1}$ exist but $(A+B)^{-1}$ doesn't.

Extending Muhammad Fuady's approach: We have: $$$$(A+B)^{-1} = A^{-1} + X$$$$ $$$$X = - (I + A^{-1}B)^{-1} A^{-1} B A^{-1}$$$$ So $$$$(A+B)^{-1} = A^{-1} - (I + A^{-1}B)^{-1} A^{-1} B A^{-1} \tag{1}\label{eq1}$$$$ This rearranges to: $$$$(A+B)^{-1} = (I - (I + A^{-1}B)^{-1} A^{-1} B )A^{-1} \tag{2}\label{eq2}$$$$ If we consider the part $$$$(I + A^{-1}B)^{-1}$$$$ Then, this is an inverse of a sum of two matrices, so we can use \eqref{eq2}, setting $$A=I$$ and $$B = A^{-1}B$$, this gives: $$$$(I + A^{-1}B)^{-1} = (I - (I + A^{-1}B)^{-1}A^{-1}B )$$$$ so we can substitute the LHS of this for the right hand side which appears in \eqref{eq2}, giving: $$$$(A+B)^{-1} = (I + A^{-1}B)^{-1}A^{-1} \tag{3}\label{eq3}$$$$ Which is simpler than \eqref{eq1} and is very similar to the scalar identity: $$$$\frac{1}{a+b}=\frac{1}{\left(1+\frac{b}{a}\right)a} \tag{4}\label{eq4}$$$$

The technique is useful in computation, because if the values in A and B can be very different in size then calculating $$\frac{1}{A+B}$$ according to \eqref{eq3} gives a more accurate floating point result than if the two matrices are summed.