TITLE: Exponential decay of voltage potential difference
QUESTION [11 upvotes]: Consider the following adjacency matrix of a complete graph:
$$A=(e^{-|i-j|})_{1\leq i\neq j\leq n}$$
with 0 on the diagonal. Let $D=diag\{d_1,...,d_n\}$ be the degree matrix where $d_i=\sum_{j\neq i}e^{-|i-j|}$. Then $L=D-A$ is the Laplacian. Let $L^\dagger$ be the Moore-Penrose inverse of the Laplacian. I'm interested in the following quantity
$$a_{ij}=|(e_1-e_2)^TL^\dagger(e_i-e_j)|$$
where $e_i=(0,0...,0,1,0,...0)$ with 1 on the ith coordinate. I conjecture that $a_{ij}$ will decay exponentially when both $i$ and $j$ moves away from 1 and 2. Something like $a_{ij}\leq C _1e^{-C_2\min\{i,j\}}$ where $C_1,C_2$ are some constants. From the physics point of view, $a_{ij}$ is the voltage potential difference between $i$ and $j$. It is intuitive that when they are far away from the source, 1 and 2, they should be very small given the structure of the graph.
In fact, my simulation shows that as long as $i,j\neq1,2$, $a_{ij}$ suddenly becomes extremely close to 0. There seems to be no decay, but an acute cut. This phenomenon holds for slight perturbation of $A$, keeping the decaying property. 
Is this conjecture true? How can we prove it? What is the rate of decay?
Another quantity that is also interesting is
$$\sum_{i\neq j}e^{-|i-j|}a_{ij}$$
which is the weighted average of potential differences. How can we bound this? For this quantity, I conjecture it is bounded by some constant instead of growing with $n$. The physical meaning of this quantity is the sum of all currents in each edge. 
(Update)
Enlightened by the discussion with @Abdelmalek Abdesselam below. We have the Neuman series representation:
$$a_{ij}=|(e_1-e_2)^TD^{-1/2}\sum_{k\geq0}\left(D^{-1/2}AD^{-1/2}-\alpha D^{1/2}JD^{1/2}\right)^kD^{-1/2}(e_i-e_j)|$$
where $J$ is the matrix of all 1s and $\alpha$ is some constant to be chosen. We want to choose $\alpha$ such that the power of the matrix decays fast. How can we achieve this and bound the entries of $D^{-1/2}AD^{-1/2}-\alpha D^{1/2}JD^{1/2}$? A possible choice is $\alpha=1/tr(D)$.

REPLY [3 votes]: Edit: This turns out to be quite simple. Observe that $a_{1i} / a_{2i} = q$ does not depend on $i \in \{3, 4, \ldots, n\}$. Thus, if $x_1 = 1$, $x_2 = -q$ and $x_i = 0$ for $i \in \{3, 4, \ldots, n\}$, then we clearly have $L x = c e_1 - c e_2$, where $c = \sum_{i=3}^n a_{1i}$. It follows that $$L^\dagger (e_1 - e_2) = c^{-1} x + \operatorname{const}.$$
I leave the previous version of this answer below, as it provides a way to evaluate $L^\dagger$ explicitly.


I cannot say I understand what is really going on here, but at least I have a proof that $a_{ij} = 0$ when $i, j \ge 3$. (I leave my previous comment/answer, as it contains some related stuff that is not included here.)

Notation: Every sum is a sum over $\{1, 2, \ldots, n\}$. We write $q = e^{-1}$ (and actually any $q \in (0, 1)$ will work). Given a vector $x = (x_i)$ we write
$$ \Delta x_i = x_{i+1} + x_{i-1} - 2 x_i $$
if $1 < i < n$.

Given a vector $(x_i)$, we have
$$ Lx_i = \sum_j q^{|i-j|} (x_i - x_j) = b_i x_i - \sum_j q^{|i-j|} x_j ,$$
where
$$ b_i = \sum_j q^{|i-j|} = \frac{1 + q - q^i - q^{n+1-i}}{1 - q} $$
Therefore, when $1 < i < n$, we have
$$ \begin{aligned}
 \Delta Lx_i & = \Delta (b x)_i - \sum_j (q^{|i-j+1|}+q^{|i-j-1|}-2q^{|i-j|}) x_j \\
 & = \Delta (b x)_i - (q + q^{-1} - 2) \sum_j q^{|i-j|} x_j + (q^{-1} - q) x_i \\
 & = \Delta (b x)_i + (q + q^{-1} - 2) L x_i - (q + q^{-1} - 2) b_i x_i + (q^{-1} - q) x_i \\
 & = (q + q^{-1} - 2) L x_i + b_{i+1} x_{i+1} + b_{i-1} x_{i-1} - ((q + q^{-1}) b_i - (q^{-1} - q)) x_i .
\end{aligned} $$
A short calculation shows that
$$ b_{i+1} + b_{i-1} = ((q + q^{-1}) b_i - (q^{-1} - q)) $$
(which looks somewhat miraculous, but there must be some insightful explanation for that). Thus,
$$ \Delta Lx_i = (q + q^{-1} - 2) L x_i + b_{i+1} (x_{i+1} - x_i) + b_{i-1} (x_{i-1} - x_i) . $$
Suppose that $x_i = L^\dagger y_i$ for some vector $(y_i)$ such that $\sum_i y_i = 0$. Then $L x_i = L L^\dagger y_i = y_i$. Write $c = q + q^{-1} - 2$. We then have
$$ \Delta y_i - c y_i = b_{i+1} (x_{i+1} - x_i) + b_{i-1} (x_{i-1} - x_i) . $$
In particular, the following claim follows.

Proposition 1: If $1 < i < n$, $y_{i-1} = y_i = y_{i+1} = 0$ and $x_i = x_{i+1}$, then $x_{i-1} = x_i$.

The above result will serve as an induction step. To initiate the induction, we need to study the $i = n$, which is slightly different. In that case:
$$ \begin{aligned}
 Lx_{n-1} - Lx_n & = b_{n-1} x_{n-1} - b_n x_n - \sum_j (q^{|n-j-1|}-q^{|n-j|}) x_j \\
 & = b_{n-1} x_{n-1} - b_n x_n - (q^{-1} - 1) \sum_j q^{|n-j|} x_j + (q^{-1} - q) x_n \\
 & = b_{n-1} x_{n-1} - b_n x_n + (q^{-1} - 1) L x_n - (q^{-1} - 1) b_n x_n + (q^{-1} - q) x_n \\
 & = (q^{-1} - 1) L x_n + b_{n-1} x_{n-1} - (q^{-1} b_n - (q^{-1} - q)) x_n .
\end{aligned} $$
This time we have
$$ q^{-1} b_n - (q^{-1} - q) = b_{n-1} , $$
and hence
$$ Lx_{n-1} - Lx_n = (q^{-1} - 1) L x_n + b_{n-1} (x_{n-1} - x_n) . $$
Again we consider $x_i = L^\dagger y_i$ for some vector $(y_i)$ such that $\sum_i y_i = 0$, and we write $d = q^{-1} - 1$. We then have
$$ (y_{n-1} - y_n) - d y_n = b_{n-1} (x_{n-1} - x_n) . $$
As a consequence, we have the following result.

Proposition 2: If $y_{n-1} = y_n = 0$, then $x_{n-1} = x_n$.

For $y = e_1 - e_2 = (1, -1, 0, 0, \ldots)$, we immediately obtain the desired result.

Corollary: If $y = e_1 - e_2$ and $x = L^\dagger y$, then $x_3 = x_4 = x_5 = \ldots = x_n$. Consequently, $a_{ij} = x_i - x_j = 0$ whenever $i, j \ge 3$.


Another consequence of the above result is that if $L^\dagger = (u_{ij})$, then $$u_{i+1,j+1}-u_{i,j+1}-u_{i+1,j}+u_{i,j} = 0$$ whenever $i + 1 < j$ or $j + 1 < i$. Moreover, it should be relatively easy to use Propositions 1 and 2 to evaluate $u_{ij}$ explicitly, and in particular to prove that
$$ u_{ij} = v_{\max\{i,j\}} + v_{\max\{n+1-i,n+1-j\}} + \tfrac{1}{4} |i - j| $$
when $i \ne j$, where $(v_i)$ is an explicitly given vector (in terms of products/ratios of $b_i$, I guess).

Final remark: there is a corresponding result in continuous variable: the Green function for the operator $$L f(x) = \int_0^1 e^{-q |x - y|} (f(x) - f(y)) dy$$ has zero mixed second-order derivative. The proof follows exactly the same line, and is in fact somewhat less technical.