58 lines
5.5 KiB
TeX
58 lines
5.5 KiB
TeX
%root:main.tex
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\subsection{Multiple Distinct $\prob$ Values}
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We would like to argue for a compressed version of $\poly(\vct{w})$, in general $\expct_{\vct{w}}\pbox{\poly(\vct{w})}$ cannot be computed in linear time.
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\AR{Added the hardness result below.}
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Our hardness result is based on the following hardness result:
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\begin{Theorem}[\cite{k-match}]
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\label{thm:k-match-hard}
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Given a positive integer $k$ and an undirected graph $G$ with no self-loops or parallel edges, counting the number of $k$-matchings in $G$ is $\#W[1]$-hard.
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\end{Theorem}
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The above result means that we cannot hope to count the number of $k$-matchings in $G=(V,E)$ in time $f(k)\cdot |V|^{O(1)}$ for any function $f$. In fact, all known algorithms to solve this problem take time $|V|^{\Omega(k)}$.
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To prove our hardness result, consider a graph $G(V, E)$, where $|E| = \numedge$, $|V| = \numvar$, and $i, j \in [\numvar]$.
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Consider the query $\poly_{G}(\vct{X}) = q_E(X_1,\ldots, X_\numvar) = \sum\limits_{(i, j) \in E} X_i \cdot X_j$.
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For the following discussion, set $\poly_{G}^\kElem(\vct{X}) = \left(q_E(X_1,\ldots, X_\numvar)\right)^\kElem$.
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\begin{Lemma}\label{lem:qEk-multi-p}
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Given polynomial $\poly_{G}^\kElem(\prob,\ldots, \prob)$, we can write $\rpoly_{G}^\kElem$ as $\rpoly_{G}^\kElem(\prob,\ldots, \prob) = \sum\limits_{i = 0}^{2\kElem} c_i \cdot \prob^i$ for some fixed terms $\vct{c}$. Given $2\kElem + 1$ distinct $\prob$ values, one can compute $\rpoly_{G}^\kElem$ for the number of $\kElem$-matchings in $G$ in $poly(\kElem)$ time.
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\end{Lemma}
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\begin{proof}[Proof of ~\cref{lem:qEk-multi-p}]
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%It is trivial to see that one can readily expand the exponential expression by performing the $n^\kElem$ product operations, yielding the polynomial in the sum of products form of the lemma statement. By definition $\rpoly_{G}^\kElem$ reduces all variable exponents greater than $1$ to $1$. Thus, a monomial such as $X_i^\kElem X_j^\kElem$ is $X_iX_j$ in $\rpoly_{G}^\kElem$, and the value after substitution is $p_i\cdot p_j = p^2$. Further, that the number of terms in the sum is no greater than $2\kElem + 1$, can be easily justified by the fact that each edge has two endpoints, and the most endpoints occur when we have $\kElem$ distinct edges (such a subgraph is also known as a $\kElem$-matching), with non-intersecting points, a case equivalent to $p^{2\kElem}$.
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Since $\rpoly_{G}^\kElem(\prob,\ldots, \prob) = \sum\limits_{i = 0}^{2\kElem} c_i \cdot \prob^i$, this implies that $\rpoly_{G}^\kElem$ is a polynomial of degree $2\kElem$ and hence $\rpoly_{G}^\kElem(\prob,\ldots, \prob)$ is a polynomial in $\prob$ of degree $2\kElem$.
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Given that we then have $2\kElem + 1$ distinct values of $\rpoly_{G}^\kElem(\prob,\ldots, \prob)$, it follows that we then have $2\kElem + 1$ linear equations which are distinct. Further, by construction of the summation, the coefficient matrix of the $2\kElem + 1$ equations is the Vandermonde matrix, from which it follows that we have a matrix with full rank, and we can solve the linear system in $O(k^3)$ time to determine $\vct{c}$ exactly.
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Note that $c_{2\kElem}$ is $\kElem! \cdot \numocc{G}{\kmatch}$. This can be seen intuitively by looking at the original factorized representation $\poly_{G}^\kElem(\vct{X})$, where, across each of the $\kElem$ products, an arbitrary $\kElem$-matching can be selected $\prod_{i = 1}^\kElem \kElem = \kElem!$ times. Note that each $\kElem$-matching $(X_{i_1} X_{j_1})\ldots$ $(X_{i_k} X_{j_k})$ corresponds to the unique monomial $\prod_{\ell = 1}^\kElem X_{i_\ell}X_{j_\ell}$ in $\rpoly_{G}^\kElem(\vct{X})$, where each index is distinct. Since $\rpoly$ contains only exponents $e \leq 1$, the only degree $2\kElem$ terms that can exist in $\rpoly_{G}^\kElem$ are $\kElem$-matchings since every other monomial in $\rpoly_{G}^\kElem(\vct{X})$ has degree $< 2\kElem$.
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%It has already been established above that a $\kElem$-matching ($\kmatch$) has coefficient $c_{2\kElem}$. As noted, a $\kElem$-matching occurs when there are $\kElem$ edges, $e_1, e_2,\ldots, e_\kElem$, such that all of them are disjoint, i.e., $e_1 \neq e_2 \neq \cdots \neq e_\kElem$. In all $\kElem$ factors of $\poly_{G}^\kElem(\vct{X})$ there are $k$ choices from the first factor to select an edge for a given $\kElem$ matching, $\kElem - 1$ choices in the second factor, and so on throughout all the factors, yielding $\kElem!$ duplicate terms for each $\kElem$ matching in the expansion of $\poly_{G}^\kElem(\vct{X})$.
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Then, since we have $\kElem!$ duplicates of each $\kElem$-matching in $\numocc{G}{\kmatch}$, $c_{2\kElem} = \kElem!\cdot\numocc{G}{\kmatch}$. This allows us to solve for $\numocc{G}{\kmatch}$ by simply dividing $c_{2\kElem}$ by $\kElem!$. By ~\cref{thm:k-match-hard} it follows then that computing $\rpoly(\vct{X})$ given multiple distinct $\prob$ values is $\#W[1]$-hard.
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\end{proof}
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\qed
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%\begin{Corollary}\label{cor:lem-qEk}
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%One can compute $\numocc{G}{\kmatch}$ in $\query_{G}^\kElem(\vct{X})$ exactly.
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%\end{Corollary}
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%
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%\begin{proof}[Proof for Corollary ~\ref{cor:lem-qEk}]
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%By ~\cref{lem:qEk-multi-p}, the term $c_{2\kElem}$ can be exactly computed. Additionally we know that $c_{2\kElem}$ can be broken into two factors, and by dividing $c_{2\kElem}$ by the factor $\kElem!$, it follows that the resulting value is indeed $\numocc{G}{\kmatch}$.
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%\end{proof}
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%
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%\qed
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%\begin{Corollary}\label{cor:tilde-q-hard}
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%Computing $\rpoly(\vct{X})$ is $\#W[1]$-hard.
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%\end{Corollary}
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%
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%\begin{proof}[Proof of Corollary ~\ref{cor:tilde-q-hard}]
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%The proof follows by ~\cref{thm:k-match-hard}, ~\cref{lem:qEk-multi-p} and ~\cref{cor:lem-qEk}.
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%\end{proof}
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