paper-BagRelationalPDBsAreHard/exact.tex

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% -*- root: main.tex -*-
\section{Exact Results}
\label{sec:exact}
We turn to computing the exact values of $\sum\limits_{\wVec, \wVecPrime \in \pw} \sketchJParam{\sketchHashParam{\wVec}} \cdot \sketchPolarParam{\wVecPrime}.$ Starting with the term $\gIJ = \sum\limits_{\wVecPrime \in \pw}\sketchPolarParam{\wVecPrime}$, by the definition of $\sketchPolar$ and the property of associativity in addition, we can break the sum into
\begin{equation*}
\gIJ = \sum_{\substack{\wVecPrime \in \pw \st\\
\sketchPolarParam{\wVecPrime} = 0}} 1 + \sum_{\substack{\wVecPrime \in \pw \st\\
\sketchPolarParam{\wVecPrime} = 1}} -1.
\end{equation*}
Setting the terms to $T_1 = \sum_{\substack{\wVecPrime \in \pw \st\\
\sketchPolarParam{\wVecPrime} = 0}} 1$ and $T_2 = \sum_{\substack{\wVecPrime \in \pw \st\\
\sketchPolarParam{\wVecPrime} = 1}} -1$ and fixing $\buck$ to a specific value, gives a system of linear equations for each term. It is a known result for a consistent multiplication that the number of solutions are $| \kDom |^{\numTup - rank(\matrixH')}$. This gives us an exact calculation for both terms,
\begin{align*}
T_1 \in \{0, 2^{\numTup - rank(\matrixH')}\},\\
T_1 \in \{0, 2^{\numTup - rank(\matrixH')}\}.
\end{align*}
\subsection{Algorithm for $\gIJ$}
\begin{algorithmic}
\If {$\matrixH' \cdot \wVec = j^{(0)}$ is consistent}
\If {$\matrixH' \cdot \wVec = j^{(1)}$ is consistent}
\State $\gIJ = 0$
\Else
\State $\gIJ = 2^{computeRank(\matrixH')}$
\EndIf
\ElsIf{$\matrixH' \cdot \wVec = \buck^{(1)}$ is consistent}
\State $\gIJ = 2^{computeRank(\matrixH')}$
\Else
$\gIJ = 0$
\EndIf.
\end{algorithmic}