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Definition 6.3 A mapping $\phi: V \to \{1, \ldots, p\}$ is an acyclic coloring if
$\phi$ is a distance-1 coloring
every cycle uses at least 3 colors
Theorem 4.6 Let $A$ be a symmetric matrix with nonzero diagonal elements, $G$ be its adjacency matrix. A mapping $\phi$ is an acyclic coloring of the adjacency graph iff it induces a substitutable partition of the columns of $A$.
Name acyclic coloring comes from the fact that the subgraph induced by any pair of colors is a forest.
To compute an off-diagonal entry $A_{ij}$, consider the subgraph induced by the two colors $\phi(i)$ and $\phi(j)$. The entry will be computed, from $B_{\phi(i)}$ and $B_{\phi(j)}$, possible after substitution.
Consider below left $B_\text{red}$ and right $B_\text{blue}$. The rows corresponding to diagonal entries have been omitted as we already found how to compute them in the previous slide. The rows corresponding to green nodes are in bold as they will be computed from the subgraph induced by red/green or blue/green so we can ignore them for now.
Theorem 3.5 A coloring of the columns is distance-2 in the bipartite graph iff columns of the same color are structurally orthogonal.
Let the $k$-th power $G^k$ of a graph $G$ be the graph with same nodes as $G$ and an edge in $G^k$ for each distance $k$ neighbors in $G$.
Lemma 3.7 The column intersection graph is the square of the biparted graph restricted to its column vertices.
Lemma 2.1 A coloring is distance-$k$ in $G$ iff it is distance-1 in $G^k$.
Corollary A coloring of the columns is distance-1 in the column intersection graph iff columns of the same color are structurally orthogonal.
It is not a distance-2 coloring because the nodes 2 and 3 are distance 2 neighbors. Because of this, in the first row of the second column of $AD$ we get $a_2 + a_3$ which prevents us distinguish $a_2$ or $a_3$. However, $a_2$ and $a_3$ can be obtained from the first column of $AD$ so we can retrieve all values of the Hessian from $AD$.
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