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Im interested in solving the system of equations $C_{i,j} = C_{i,j-1} + C_{i,j+1} + C_{i-1,j} + C_{i+1,j}$ for $0 \leq i < N$ and $0 \leq j < N$, where $C_{i,j} \in \mathbb{C}$. One could think of this a regular grid where each entry of a cell corresponds to the sum of the neighboring cells. Additionally i want to have periodic boundary conditions in both indices, i.e. $C_{i+N, j} = C_{i, j}$ and $C_{i, j+N} = C_{i, j}$, hence we can think of the grid mentioned before as a torus. Problem: find all possibile non-trivial solutions of this system of equations. My first attempt was to substract $- 4C_{i,j}$ on both sides such that we can think of the problem as $- 3 C_{i,j} = C_{i,j-1} - 2 C_{i,j} + C_{i,j+1} + C_{i-1,j} - 2C_{i,j} + C_{i+1,j}$ which is essentially a discrete version of theHelmholtz equation on a unit grid, i.e. $\frac{\partial^{2} C_{i,j}}{\partial i^2} + \frac{\partial^{2} C_{i,j}}{\partial j^2} + 3 C_{i,j}$. Due to lack of knowledge i did not find any other good ansatz. Has this problem already been solved?

Thanks in advance!

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One could use the fact that the associated coefficient matrix is block-circulant.

Let $\otimes$ denote the Kronecker product. Let $\mathbf e_0, \mathbf e_1,\dots,\mathbf e_{N-1}$ denote the standard basis of $\Bbb R^{N}$. Let $P$ denote the size-$N$ permutation matrix $$ P = \pmatrix{0&\cdots & 0 & 1\\1&0&\cdots&0\\\vdots & \ddots & \ddots & \vdots\\ 0 & \cdots & 1 & 0}. $$ Let $\mathbf c$ denote the column-vector $$ \mathbf c = (C_{0,0},C_{0,1},\dots,C_{0,N},C_{1,0},C_{1,1},\dots) = \sum_{i,j = 0}^{N -1} C_{ij}\, \mathbf e_i \otimes \mathbf e_j. $$ Your system of equations can be written in the form $M \mathbf c = 0$, where $$ M = I_{N} \otimes (P + P^T) + (P + P^T) \otimes I_{N} - I_{N^2}. $$ We see that this matrix can be diagonalized. In particular, we have $$ (F \otimes F) M (F \otimes F)^* = I_{N} \otimes D + D \otimes I_{N} - I_{N^2}, $$ where $D$ is the diagonal matrix $$ D = \operatorname{diag}(2\cos(0 \cdot \theta),2\cos(1 \cdot \theta), \dots, 2 \cos((N-1) \cdot \theta)), \quad \theta = \frac{2 \pi }{N}. $$


An alternative, simpler approach. Use the Ansatz $$ C_{p,q} = \omega_1^p \omega_2^q $$ where $\omega_1,\omega_2$ are complex numbers. We have $$ C_{p,q} = C_{p,q-1} + C_{p,q+1} + C_{p-1,q} + C_{p+1,q} \implies\\ \omega_1^p\omega_2^q = \omega_1^p\omega_2^{q+1} + \omega_1^p\omega_2^{q-1} + \omega_1^{p+1}\omega_2^q + \omega_1^{p-1}\omega_2^q \implies\\ \omega_1^p \omega_2^q = (\omega_1 + \omega_1^{-1} + \omega_2 + \omega_2^{-1})\omega_1^p \omega_2^q \implies\\ \omega_1 + \omega_1^{-1} + \omega_2 + \omega_2^{-1} = 1. $$ Now, due to the periodic boundary condition, we must have $\omega_1^{N} = \omega_2^{N} = 1$, so that we have $$ \omega_j = \exp(i k_j\theta), \quad \theta = \frac{2 \pi}{N}, \quad 0 \leq k_j \leq N-1, \quad j = 1,2. $$ For this value of $\theta$, there is a non-trivial solution to the system for every solution to the equation $$ \omega_1 + \omega_1^{-1} + \omega_2 + \omega_2^{-1} = 1 \implies\\ 2[\cos(k_1 \theta) + \cos(k_2 \theta)] = 1. $$ As can be shown using the matrix representation of the system from the first part, the system will have a solution if and only if this system has a solution for some integer values $k_1,k_2$.

An alternative formulation: let $\alpha = \theta/2$. With the sum to product formula, we can rewrite this as $$ \cos([k_1 + k_2]\alpha)\cos([k_1 - k_2]\alpha) = \frac 14. $$

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