# Poisson equation finite-difference with pure Neumann boundary conditions

I'm trying to solve a 1D Poisson equation with pure Neumann boundary conditions. I've found many discussions of this problem, e.g.

And many more.

Some of the answers seem unsatisfactory though. For example, the answer in 2) contains a document that explains how the matrix $A$ changes when applying Neumann boundary conditions, but does not explain how to solve the singular system. The comment left in 2) by @Evgeni Sergeev is a reference to a problem with mixed boundary conditions, and not pure Neumann boundary conditions.

That said, there are some useful things I've found. I do like the second comment given in the 1) by @Sumedh Joshi. It suggests to subtract the mean from the RHS. Other references I've read suggest using Dirichlet BCs at one location in the computational domain which should result in a unique solution, however, I prefer removing the mean since this seams more elegant.

The problem setup is:

$\frac{\partial^2 u}{\partial x^2} = f, \qquad f = -(2\pi)^2 cos(2\pi x), \qquad 0 \le x\le 1, \qquad \left(\frac{\partial u}{\partial x}\right)_{0} = 0, \qquad \left(\frac{\partial u}{\partial x}\right)_{1} = 0$

I have ghost points in my computational domain, so my matrix $A$ has some extra zeros (I remove them later). Using central differences everywhere in the computational domain, matrix $A$ takes the following form:

$A = \frac{1}{\Delta x^2} \left[ \begin{array}{ccccccccc} 0 & 0 & 0 & & & & & & \\ 1 & -2 & 1 & & & & & & \\ 0 & 1 & -2 & 1 & & & & & \\ & & & \ddots & \ddots & \ddots & & & \\ & & & & & 1 & -2 & 1 & 0 \\ & & & & & & 1 & -2 & 1 \\ 0 & & & & & & 0 & 0 & 0 \\ \end{array} \right]$

Once applying the ghost points and adjusting the matrix $A$, I end up with

$A = \frac{1}{\Delta x^2} \left[ \begin{array}{ccccccccc} 0 & 0 & 0 & & & & & & \\ 0 & -2 & 2 & & & & & & \\ 0 & 1 & -2 & 1 & & & & & \\ & & & \ddots & \ddots & \ddots & & & \\ & & & & & 1 & -2 & 1 & 0 \\ & & & & & & 2 & -2 & 0 \\ 0 & & & & & & 0 & 0 & 0 \\ \end{array} \right]$

Correspondingly, the RHS has changed to

$\frac{- 2u_b + 2u_i}{\Delta x^2} = f_b - \frac{-2\theta \Delta x}{\Delta x^2}$

Where $u_b,u_i,f_b,\theta$ are the $u$ at the boundary, $u$ at the first interior point, $f$ at the boundary and the slope of the solution at the boundary (zero in this case) respectively. The interior of this matrix is the same as the one discussed in the document given in the accepted answer in 2).

Here I show the setup and results of a small MATLAB script where I print out $A, f, mean(f)$ and the (attempted) solution $u$ using MATLAB's backslash operator.

  A =

-100.0000  100.0000         0         0         0         0         0         0         0         0         0
100.0000 -200.0000  100.0000         0         0         0         0         0         0         0         0
0  100.0000 -200.0000  100.0000         0         0         0         0         0         0         0
0         0  100.0000 -200.0000  100.0000         0         0         0         0         0         0
0         0         0  100.0000 -200.0000  100.0000         0         0         0         0         0
0         0         0         0  100.0000 -200.0000  100.0000         0         0         0         0
0         0         0         0         0  100.0000 -200.0000  100.0000         0         0         0
0         0         0         0         0         0  100.0000 -200.0000  100.0000         0         0
0         0         0         0         0         0         0  100.0000 -200.0000  100.0000         0
0         0         0         0         0         0         0         0  100.0000 -200.0000  100.0000
0         0         0         0         0         0         0         0         0  100.0000 -100.0000

F =

-19.7392
-31.9387
-12.1995
12.1995
31.9387
39.4784
31.9387
12.1995
-12.1995
-31.9387
-19.7392

meanF =

-9.6892e-016

Warning: Matrix is singular to working precision.
> In main at 121

u =

NaN
NaN
NaN
NaN
NaN
NaN
NaN
NaN
NaN
-Inf
-Inf


My question is why is this not working? I thought that removing the mean of the RHS would work as suggested in @Sumedh Joshi's comment. Any help is greatly appreciated.

UPDATE:

I found out that this same exact problem setup works perfectly fine for 12 unknowns (and 15 and much larger, e.g. 100), but does not work for 10 unknowns (as posted). So I suppose that the problem is setup correctly. This still begs the question though, what is going on here? It seems that this may be more of a question regarding numerical analysis.

Here are the matlab results for 12 unknowns:

 A =
0         0         0         0         0         0         0         0         0         0         0         0         0         0         0
0   -1.0    1.0         0         0         0         0         0         0         0         0         0         0         0         0
0    1.0   -2.0    1.0         0         0         0         0         0         0         0         0         0         0         0
0         0    1.0   -2.0    1.0         0         0         0         0         0         0         0         0         0         0
0         0         0    1.0   -2.0    1.0         0         0         0         0         0         0         0         0         0
0         0         0         0    1.0   -2.0    1.0         0         0         0         0         0         0         0         0
0         0         0         0         0    1.0   -2.0    1.0         0         0         0         0         0         0         0
0         0         0         0         0         0    1.0   -2.0    1.0         0         0         0         0         0         0
0         0         0         0         0         0         0    1.0   -2.0    1.0         0         0         0         0         0
0         0         0         0         0         0         0         0    1.0   -2.0    1.0         0         0         0         0
0         0         0         0         0         0         0         0         0    1.0   -2.0    1.0         0         0         0
0         0         0         0         0         0         0         0         0         0    1.0   -2.0    1.0         0         0
0         0         0         0         0         0         0         0         0         0         0    1.0   -2.0    1.0         0
0         0         0         0         0         0         0         0         0         0         0         0    1.0   -1.0         0
0         0         0         0         0         0         0         0         0         0         0         0         0         0         0

f =

0
-19.7392
-34.1893
-19.7392
-0.0000
19.7392
34.1893
39.4784
34.1893
19.7392
0.0000
-19.7392
-34.1893
-19.7392
0

mean(f)
ans =
-9.4739e-016

A(2:end-1,2:end-1)\f(2:end-1)
ans =
0.9167
0.7796
0.4051
-0.1065
-0.6181
-0.9926
-1.1297
-0.9926
-0.6181
-0.1065
0.4051
0.7796
0.9167

• Is this equation meant to be nonlinear? Or is that 'u' next to the second derivative incorrect? – spektr Dec 18 '15 at 21:04
• Whoops! It's linear, fixed it. – Charles Dec 18 '15 at 21:20
• The matrix in code starts (top left) $[-100,100;100,-200]$, but the matrix in the formula starts $[-2,2;-1,2]/\Delta x^2$, so shouldn't it be $[-200,200;100,-200]$? – Kirill Dec 19 '15 at 6:40
• I did not mention, I've divided the first and last non-zero rows by 2 (including the RHS) so that the matrix is symmetric. The matrices should be equivalent.. – Charles Dec 19 '15 at 6:45
• I will include this in the question when I get a chance – Charles Dec 19 '15 at 6:46

First a short answer - it did not work because your matrix is singular. The matrix is singular, because you did not do all steps necessary for the case of zero Neumann boundary condition.

A longer answer - I guess your expectation it should work (I might be wrong) were due to the fact that the mean of right hand side is in your case (numerically almost) zero. There exists a step in this kind of problems where a mean of right hand side is subtracted from it, but this is not enough, and in your case it is not necessary because it is equal 0.

In fact, the zero mean of $f$ (or zero sum of entries in your $f$) means in your case that there exists a solution to your problem, in fact, there are infinitely many solutions. You still have to do some additional work to find at least one solution from those infinitely many ones.

Let me first note how to see easily that your matrix is singular. I mean the matrix in your output from Matlab, i.e. without the first and last row and column with zero entries only. If you sum all rows in your matrix $A$ you get the row with zeros only. This can happen only for singular matrix. In fact it means that you can delete e.g. the last row in your matrix, because it is redundant, it can be expressed from other rows.

Luckily, if you do the same with the right hand side $f$ then you get also the zero (it means also the mean of $f$ is zero). If this would not be case, your system has no solution (a brief explanation - it would be like you have the zero row in the matrix with non zero right hand side, such equation can not be fulfilled).

So now what to do to find at least one representative solution. In general you must add some additional equation that makes the solution unique. It means you have to add one row in the matrix and corresponding one right hand side.

One option you mentioned in your question (and it is the most typical one) is to say e.g. that the last unknown (for which in fact you erased the row in $A$) you prescribe an arbitrary value. It is like you replace the zero Neumann boundary condition with Dirichlet one. So it is like to add the row $(0 \, \ldots \, 0 \, 1)$ to replace the erased one.

Another one is that you require that the mean of unknowns has a prescribed value, e.g. zero. It is like you have to add a row with ones only, i.e. $(1 \, \ldots 1)$ to replace the erased row. That is a reason that such option is not very popular, because it changes the structure of your matrix.

• It doesn't seem to me that this answer offers anything new to what I've already wrote in my question and included in links, with the exception of your last paragraph. Also, you haven't explained how one could ensure that the mean of the unknowns is zero. Could you please include a reference to this method you describe in the last paragraph? Also, I've included an update in my question that might help better explain what is wrong. – Charles Feb 2 '16 at 17:59

As stated by Peter the matrix of the all Neumann boundary Poisson problem is indeed singular and therefore the Matlab Backslash operator, which attempts to use direct solvers, cannot deal with it. Conjugate Gradient on the other hand can, you just need to, as you have, ensure the mean of f equals 0 before handing it to e.g. PCG.

I would like to stress that arbitrarily replacing a Neumann boundary with a fixed (arbitrary) valued Dirichelt boundary is not the correct way of dealing with the problem.

To demonstrate this, here is some code for anyone to verify in Matlab or Octave:

clear;

n = 64;
lx = 1.0;
BCA = 0; %-0.1533;

% problem setup
dx=lx/(n-1);
x=0:dx:1;

% set up A's to compute Laplacian on internal points
nx=n-2;
e=ones(nx-2,1);
% left Dirichelt, right Neumann
AD = - spdiags([1 -2 1; e*[1 -2 1]; 1 -1 1],-1:1,nx,nx)/dx^2;
% both sides Neumann
AN = - spdiags([1 -1 1; e*[1 -2 1]; 1 -1 1],-1:1,nx,nx)/dx^2;

% set up RHS for internal points
b=zeros([nx 1]);
% b(1:nx)=4*sin(33/pi.*x(2:nx+1));
b(1:nx)=-2*pi*cos(2*pi.*x(2:nx+1));

% incorporate left Dirichelt BC conditon into RHS
% e.g. p.2 of https://web.stanford.edu/class/cs205b/lectures/lecture16.pdf
bD=b;

% ensure Discrete Compatibility for Neumann case, i.e. mean(bN)=0
bN=b-mean(b);

% solve both with CG
xN=pcg(AN,bN,1e-6,2*n);

% solutions are obtained on internal points only and boundary conditions
% are incorporated into the RHS. For plotting values need to be appended.
xD=[ BCA ; xD; xD(end)];
xN=[xN(1); xN; xN(end)];

% compute first derivatives, frequently of interest for potential problems
% and also to see the Neumann boundary conditions

% plot solutions
f=figure(1); set(f,'Position', [200, 300, 720, 310]);
subplot(1,2,1);
plot(x,xD,'r-'); hold on
plot(x,xN,'b--'); hold off
title('Solutions'); grid on;

subplot(1,2,2);
plot(x,dxDdx,'r-'); hold on
plot(x,dxNdx,'b--'); hold off
title('First Derivatives of Solutions'); grid on;
% legend('left Dirichelt','both Neumann','location','SouthEast');
legend('left Dirichelt','both Neumann','location','NorthEast');


This will produce the following output:

One might be inclined to think that fixing the left boundary and thus being able to use the Backslash operator has done the job. The agreement between the two is good enough, however for a different rhs the results can differ significantly. Changing the rhs to e.g.:

b(1:nx)=4*sin(33/pi.*x(2:nx+1));


Produces the following output:

Here both the solutions and their first derivatives are very different, especially near where the boundary condition of the problem has been changed.

There are many different ways of dealing with singular matrices, but using an arbitrary Dirichlet boundary condition is not one. For the problem at hand it is easiest to switch to an iterative solver like PCG and ensure Discrete Compatibility.