# Is there any method to calculate the inverse of matrix which changed a few values?

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I have a large sparse matrix A and have gotten its inverse matrix inv(A) . Then I need to change an element value to get a new matrix, A1. I am trying to get the inverse of A1. Is there any way to do it , rather than recalculate the inv(A1)? Could I get some benefits from A or inv(A)?

e.g. (not a sparse matrix)

A = magic(5);

invA = inv(A);

A1 = A;

A1(1,5) = 1;

invA1 = inv(A1);% is there any way to do this?

I know this may be a math problem. It seems impossible for me. I just would like to know other's viewpoints of it. Thank you.

##### 0 Comments

### Accepted Answer

John D'Errico
on 1 Aug 2020

Edited: John D'Errico
on 1 Aug 2020

Bruno is correct in how to solve the problem, and also SOME of the issues with it. If you use this tool multiple times, the floating point errors will begin to accumulate.

There is however, still a serious problem. That is time. I recall looking at this formula long ago, when I first learned about it. (35 - 40 years ago.) At the time, I concluded it might not be such a great tool, both due to the numerical issues, as well as the time required to perform the computation.

While the idea of a rank 1 update for the inverse of a matrix seems interesting, it has serious problems in practice. Gosh, it must be more efficient that just recomputing the inverse of the matrix, right? Always test these things.

I ran a test of this code. In my tests, I computed a random matrix of size N, then the inverse of that matrix. Next, I chose a random element to modify to some new random value.

Finally, I used timeit to measure the time required for a matrix inverse of that matrix. Then I used timeit to measure the time required for this update to the matrix inverse. The results are found in the table below.

N invtime updatetime update ratio

____ __________ __________ ____________

50 4.6589e-05 9.3269e-05 2.0019

100 0.00012982 0.00048521 3.7375

200 0.0004016 0.0018684 4.6524

400 0.0014756 0.0073921 5.0094

600 0.0041071 0.017325 4.2183

800 0.0070771 0.032447 4.5848

1000 0.012438 0.055685 4.4771

1300 0.025226 0.11205 4.4419

1600 0.045956 0.31944 6.9509

1900 0.075096 0.81713 10.881

2200 0.11136 1.6726 15.019

2500 0.16798 3.0376 18.083

3000 0.29141 6.2889 21.581

So column 1 in that table is the order of the matrix used in the test.

invtime is the time used to compute one matrix inverse of a matrix of that size.

updatetime is the time required for the function updateinv to do its work. As you should see, updateinv is considerably SLOWER than a simple, direct inverse of the matrix.

The final column is the relative cost. As you should see, updateinv actually starts to get worse as the matrix size itself grows.

In the loglog plot, you should see that the matrix inverse itself takes consistently LESS time to perform. As well, you can see the differential gets worse for larger matrices.

If you don't trust the data I provide, run a test yourself. Even a 2Kx2K matrix inverse is not that costly to compute.

A = rand(2000);

timeit(@() inv(A))

ans =

0.084881

As you can see, this mirrors the numbers in column 2 in my table. I stopped at 3K square matrices, because the time required for the update was starting to get expensive, and I wanted to post this response.

I've attached the script I used to do these time tests, in case you wish to verify my results.

So you seriously need to consider if this is a good idea. While I always strongly advise considering if you even want to compute the matrix inverse at all as there are better things to do almost always, updating that inverse using the code posted by Bruno was never a savings in time. If you will perform multiple updates, then you are further behind, since now you will also incur penalties due to accumulation of the errors.

Finlly, should Bruno wish to soend some time in optimizing his code, I would add that the profile tool tells me most of the time required seems to be in just in creating the tolerance.

A = rand(2000);

timeit(@() norm(A))

ans =

1.0575

timeit(@() normest(A))

ans =

0.011862

norm(A)

ans =

1000.1

normest(A)

ans =

1000.1

So, if I just modify one line from the code written by Bruno, replacing norm with a call to normest,

function [Amod, iAupdate] = updateinv(A, i, j, newAij, iA)

% INPUTS:

% A (n x n) matrix

% i, j, newAij are scalars: A(i,j) will change to newAij

% iA inverse of A

% OUTPUTS:

% Amod: n x n matrix subjected to this modification

% iAupdate: inverse of Amod, updated using Shermanâ€“Morrison formula

Amod = A;

Amod(i,j) = newAij;

d = newAij-A(i,j);

c = 1+d*iA(j,i);

tol = max(size(A)) * eps(normest(A));

if abs(c) < tol

warning('update matrix might be inaccurate');

if c > 0

c = tol;

else

c = -tol;

end

end

iAupdate = iA - d/c * (iA(:,i)*iA(j,:));

end

the times required have now inverted in their behavior.

N invtime updatetime update ratio

____ __________ __________ ____________

50 4.0205e-05 9.2221e-06 0.22938

100 0.0001385 3.005e-05 0.21697

200 0.00039433 7.6963e-05 0.19517

400 0.0018709 0.00019188 0.10256

600 0.0040269 0.0012491 0.31018

800 0.0071675 0.0025266 0.3525

1000 0.012501 0.0052236 0.41786

1300 0.023901 0.011431 0.47824

1600 0.044222 0.01764 0.3989

1900 0.073962 0.024691 0.33384

2200 0.10989 0.033139 0.30156

2500 0.17115 0.043476 0.25402

3000 0.29513 0.063203 0.21415

The cost of using normest is it is only an approximate estimate of the norm of A. But here we see we could have done several updates to the inverse in the time required for one inverse. The question of accumulation of errors is another issue, worth some study in itself.

##### 2 Comments

Bruno Luong
on 1 Aug 2020

Edited: Bruno Luong
on 1 Aug 2020

Explicit compute INV is raeely the good idea. We see that the 1-rank update is reduces to something like this:

iAupdate = iA + a * u * v';

where a is scalar and u and v' are (n x 1) vector

Down of the line, at the end of the day this inverse has to be multiplied with some vector x, so

iAupdate*x = iA*x + a * u * v' * x.

Well the most economic to perform that is in this order:

iAupdate*x = iA*x + (a * (v' * x)) * u.

So actually the update of matrix-vector product need O(n) operations where as update explicitly the inverse-matrix needs O(n^2).

There we go, it explains why it is also not terribly useful for CPU as John has pointed out.

This kind of formula (applied to matrix-vector product) gives rise for BFGS formula, Kamal filtering, Householder transformation, etc... which better to be mastered and applied for each specific problem.

### More Answers (2)

Vladimir Sovkov
on 1 Aug 2020

I am not sure how much profitable it is numerically, but the Sherman Morrison theorem can be a way, see https://en.wikipedia.org/wiki/Sherman%E2%80%93Morrison_formula

In your example, u vector there should be chosen with only the 1st nonzero element and v vector with only the 5th nonzero element so that u1*v5=(1-A15).

##### 2 Comments

Vladimir Sovkov
on 1 Aug 2020

Bruno Luong
on 1 Aug 2020

Edited: Bruno Luong
on 1 Aug 2020

Apply this Sherman Morisson's formula in the page provided by Vladimir, if one set a single change: A(i,j) to new value newAij, the update should goes like this

function [Amod, iAupdate] = updateinv(A, i, j, newAij, iA)

% INPUTS:

% A (n x n) matrix

% i, j, newAij are scalars: A(i,j) will change to newAij

% iA inverse of A

% OUTPUTS:

% Amod: n x n matrix subjected to this modification

% iAupdate: inverse of Amod, updated using Sherman–Morrison formula

Amod = A;

Amod(i,j) = newAij;

d = newAij-A(i,j);

c = 1+d*iA(j,i);

tol = max(size(A)) * eps(norm(A));

if abs(c) < tol

warning('update matrix might be inaccurate');

if c > 0

c = tol;

else

c = -tol;

end

end

iAupdate = iA - d/c * (iA(:,i)*iA(j,:));

end

Test script

% testupdate.m

% Random matrix test

A = rand(5);

iA = inv(A);

% Random change

i = randi(size(A,1));

j = randi(size(A,2));

newAij = 10;

% Update matrix and its inverse

[Amod, iAupdate] = updateinv(A, i, j, newAij, iA);

% Check

iAmod = inv(Amod)

iAupdate

relerr = norm(iAupdate-iAmod,'fro')/norm(iAmod,'fro')

Result:

>> testupdate

iAmod =

0.3427 -0.2397 0.0926 -0.1965 -0.0649

-3.1551 3.3784 0.1696 1.3277 -0.5608

-4.7509 1.9002 0.1144 2.6354 0.9978

5.2877 -3.8865 -0.3091 -2.5948 1.2505

-1.4484 1.5561 0.1196 1.7662 -1.5276

iAupdate =

0.3427 -0.2397 0.0926 -0.1965 -0.0649

-3.1551 3.3784 0.1696 1.3277 -0.5608

-4.7509 1.9002 0.1144 2.6354 0.9978

5.2877 -3.8865 -0.3091 -2.5948 1.2505

-1.4484 1.5561 0.1196 1.7662 -1.5276

relerr =

4.0984e-16

From my experience this formula is not terribly useful for 2 reasons

- I rarely store inverse of matrix, epsecially medium/large size.
- One can lost precision when apply this formula multiple times.

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