From 86b8f967a12fe5870fe7c8d0f765149c003832c6 Mon Sep 17 00:00:00 2001
From: Stratis Ioannidis <stratis@stratis-Latitude-E6320.(none)>
Date: Sun, 4 Nov 2012 16:44:07 -0800
Subject: style stuff

---
 main.tex | 87 +++++++++++++++++++++++++++++-----------------------------------
 1 file changed, 40 insertions(+), 47 deletions(-)

(limited to 'main.tex')

diff --git a/main.tex b/main.tex
index 0448dab..560a86a 100644
--- a/main.tex
+++ b/main.tex
@@ -107,15 +107,14 @@ Here, following these ideas, we introduce a relaxation specifically tailored to
 function of \EDP. The multi-linear extension of \eqref{modified} can be written:
 \begin{displaymath}
     F_\mathcal{N}(\lambda) = \mathbb{E}_{S\sim
-    P_\mathcal{N}^\lambda}\left[\log\det A(S) \right]
-    \;\text{with}\; A(S) = I_d + \sum_{i\in S} x_i\T{x_i}
+    P_\mathcal{N}^\lambda}\Big[\log\det \big(I_d + \sum_{i\in S} x_i\T{x_i}\big) \Big].
 \end{displaymath}
 As in the case of \textsc{Coverage}, \eqref{relax} is not a convex optimization problem, and is not easy to solve directly. %As in ~\cite{singer-influence},
 We thus introduce an additional relaxation $L_{\mathcal{N}}$. Our relaxation follows naturally by swapping the expectation and the $\log\det$
 in the above formula:
 \begin{align}\label{eq:concave}
-    L_\mathcal{N}(\lambda) & \defeq \log\det\left(\mathbb{E}_{S\sim
-    P_\mathcal{N}^\lambda}[A(S)]\right)\notag \\
+    L_\mathcal{N}(\lambda) & \defeq \log\det\Big(\mathbb{E}_{S\sim
+    P_\mathcal{N}^\lambda}\big[\log\det (I_d + \sum_{i\in S} x_i\T{x_i})\big]\Big)\notag \\
     &= \log\det\left(I_d + \sum_{i\in\mathcal{N}}
     \lambda_i x_i\T{x_i}\right)
 \end{align}
@@ -128,8 +127,7 @@ be to prove that $OPT(L_\mathcal{N}, B)$ is close to $V(S_G)$. To do so, our
 main technical contribution is to prove that $L_\mathcal{N}$ has a bounded
 approximation ratio to the value function $V$ (Lemma~\ref{lemma:relaxation}).
 
-The resulting mechanism for \EDP{} is presented in Algorithm~\ref{mechanism}. The constant $C$ is an absolute constant, that we determine in Section~\ref{sec:proofofmainthm} (see \eqref{eq:C}).
-\begin{algorithm}
+\begin{algorithm}[!t]
     \caption{Mechanism for \EDP{}}\label{mechanism}
     \begin{algorithmic}[1]
     \State $i^* \gets \argmax_{j\in\mathcal{N}}V(j)$
@@ -150,6 +148,7 @@ The resulting mechanism for \EDP{} is presented in Algorithm~\ref{mechanism}. Th
         \EndIf
     \end{algorithmic}
 \end{algorithm}
+The resulting mechanism for \EDP{} is presented in Algorithm~\ref{mechanism}. The constant $C$ is an absolute constant, that we determine in Section~\ref{sec:proofofmainthm} (see \eqref{eq:c}).
 We can now state our main result:
 \begin{theorem}\label{thm:main}
     There exists an absolute constant $C$ such that the mechanism described in Algorithm~\ref{mechanism} is truthful, individiually rational 
@@ -335,26 +334,26 @@ This solution is:
     gives the approximation ratio in \eqref{approxbound}, and concludes the proof of Theorem~\ref{thm:main}.\hspace*{\stretch{1}}\qed
 %\end{proof}
 \subsection{Proof of Lemma~\ref{lemma:relaxation}}\label{sec:relaxation}
-
-Since $L_\mathcal{N}$ is a relaxation of the value function $V$, we have:
-\begin{displaymath}
-    OPT(V,\mathcal{N},B) \leq OPT(L_\mathcal{N}, B)
-\end{displaymath}
-However, for the purpose of proving theorem~\ref{thm:main}, we need to bound
-$L_\mathcal{N}$ from above (up to a multiplicative factor) by $V$. We use the
-\emph{pipage rounding} method from Ageev and Sviridenko \cite{pipage}, where
-$L_\mathcal{N}$ is first bounded from above by $F_\mathcal{N}$, which is itself
-subsequently bounded by $V$. While the latter part is general and can be apply
+%Recall that, since $L_\mathcal{N}$ is a fractional relaxation of  $V$, 
+%\begin{displaymath}
+%    OPT(V,\mathcal{N},B) \leq OPT(L_\mathcal{N}, B).
+%\end{displaymath}
+%However, for the purpose of proving theorem~\ref{thm:main}, we need to bound
+%$L_\mathcal{N}$ from above (up to a multiplicative factor) by $V$. 
+Toprove Lemma~\ref{lemma:relaxation}, we use the
+\emph{pipage rounding} method of Ageev and Sviridenko~\cite{pipage}, where
+$L_\mathcal{N}$ is first bounded from above by the multi-linear relaxation $F_\mathcal{N}$, which is itself
+subsequently bounded by $V$. While the latter part is general and can be applied
 to the multi-linear extension of any submodular function, the former part is
-specific to our choice for the function $L_\mathcal{N}$ and is our main
-technical contribution (lemma~\ref{lemma:relaxation-ratio}).
+specific to our choice for the function $L_\mathcal{N}$.  %and is our main technical contribution (lemma~\ref{lemma:relaxation-ratio}).
 
-First we prove a variant of the $\varepsilon$-convexity of the multi-linear
+We first prove a variant of the $\varepsilon$-convexity of the multi-linear
 extension \eqref{eq:multilinear} introduced by Ageev and Sviridenko
-\cite{pipage} which allows to trade a fractional component for another until
+\cite{pipage} which allows to trade one fractional component of the solution for another until
 one of them becomes integral, without loosing any value. This property is also
-referred to in the literature as \emph{cross-convexity} (see \emph{e.g.}
-\cite{dughmi}). Formally, this property states that if we define:
+referred to in the literature as \emph{cross-convexity} (see, \emph{e.g.},
+\cite{dughmi}). Formally, %this property states that 
+if we define:
 \begin{displaymath}
     \tilde{F}_\lambda(\varepsilon) \defeq F_\mathcal{N}\big(\lambda + \varepsilon(e_i
     - e_j)\big)
@@ -367,30 +366,27 @@ $\reals^{n}$, then $\tilde{F}$ is convex. Hence its maximum over the interval:
 is attained at one of the boundaries of $I_\lambda$ for which one of the $i$-th
 or the $j$-th component of $\lambda$ becomes integral.
 
-The lemma below proves that we can similarly trade one fractional component for
+The lemma below proves that we can similarly trade a fractional component for
 an other until one of them becomes integral \emph{while maintaining the
 feasibility of the point at which $F_\mathcal{N}$ is evaluated}. Here, by feasibility of
-a point $\lambda$, we mean that it satisfies the budget constraint $\sum_{1\leq
-i\leq n} \lambda_i c_i \leq B$.
-
+a point $\lambda$, we mean that it satisfies the budget constraint $\sum_{i=1}^n \lambda_i c_i \leq B$.
 \begin{lemma}[Rounding]\label{lemma:rounding}
     For any feasible $\lambda\in[0,1]^{n}$, there exists a feasible
-    $\bar{\lambda}\in[0,1]^{n}$ such that at most one of its component is
-    fractional, that is, lies in $(0,1)$ and:
-    \begin{displaymath}
-        F_{\mathcal{N}}(\lambda)\leq F_{\mathcal{N}}(\bar{\lambda})
-    \end{displaymath}
+    $\bar{\lambda}\in[0,1]^{n}$ such that at most one of its components is
+    fractional %, that is, lies in $(0,1)$ and:
+     and 
+    %\begin{displaymath}
+     $   F_{\mathcal{N}}(\lambda)\leq F_{\mathcal{N}}(\bar{\lambda})$.
+    %\end{displaymath}
 \end{lemma}
-
 \begin{proof}
-    We give a rounding procedure which given a feasible $\lambda$ with at least
-    two fractional components, returns some $\lambda'$ with one less fractional
-    component, feasible, and such that:
+    We give a rounding procedure which, given a feasible $\lambda$ with at least
+    two fractional components, returns some feasible $\lambda'$ with one less fractional
+    component such that:
     \begin{displaymath}
         F_\mathcal{N}(\lambda) \leq F_\mathcal{N}(\lambda')
     \end{displaymath}
     Applying this procedure recursively yields the lemma's result.
-
     Let us consider such a feasible $\lambda$. Let $i$ and $j$ be two
     fractional components of $\lambda$ and let us define the following
     function:
@@ -399,22 +395,20 @@ i\leq n} \lambda_i c_i \leq B$.
         \quad\textrm{where} \quad
         \lambda_\varepsilon = \lambda + \varepsilon\left(e_i-\frac{c_i}{c_j}e_j\right)
     \end{displaymath}
-
     It is easy to see that if $\lambda$ is feasible, then:
     \begin{multline}\label{eq:convex-interval}
         \forall\varepsilon\in\Big[\max\Big(-\lambda_i,(\lambda_j-1)\frac{c_j}{c_i}\Big), \min\Big(1-\lambda_i, \lambda_j
         \frac{c_j}{c_i}\Big)\Big],\;\\
             \lambda_\varepsilon\;\;\textrm{is feasible}
     \end{multline}
-
-    Furthermore, the function $F_\lambda$ is convex, indeed:
+    Furthermore, the function $F_\lambda$ is convex; indeed:
     \begin{align*}
         F_\lambda(\varepsilon)
         & = \mathbb{E}_{S'\sim P_{\mathcal{N}\setminus\{i,j\}}^\lambda(S')}\Big[
         (\lambda_i+\varepsilon)\Big(\lambda_j-\varepsilon\frac{c_i}{c_j}\Big)V(S'\cup\{i,j\})\\
         & + (\lambda_i+\varepsilon)\Big(1-\lambda_j+\varepsilon\frac{c_i}{c_j}\Big)V(S'\cup\{i\})\\
         & + (1-\lambda_i-\varepsilon)\Big(\lambda_j-\varepsilon\frac{c_i}{c_j}\Big)V(S'\cup\{j\})\\
-        & + (1-\lambda_i-\varepsilon)\Big(1-\lambda_j+\varepsilon\frac{c_i}{c_j}\Big)V(S')\Big]\\
+        & + (1-\lambda_i-\varepsilon)\Big(1-\lambda_j+\varepsilon\frac{c_i}{c_j}\Big)V(S')\Big]
     \end{align*}
     Thus, $F_\lambda$ is a degree 2 polynomial whose dominant coefficient is:
     \begin{multline*}
@@ -430,15 +424,14 @@ i\leq n} \lambda_i c_i \leq B$.
 \end{proof}
 
 \begin{lemma}\label{lemma:relaxation-ratio}
-    The following inequality holds:
-    \begin{displaymath}
-        \forall\lambda\in[0,1]^{n},\; 
-        \frac{1}{2}
+ %   The following inequality holds:
+For all $\lambda\in[0,1]^{n},$
+    %\begin{displaymath}
+     $           \frac{1}{2}
         \,L_\mathcal{N}(\lambda)\leq
-        F_\mathcal{N}(\lambda)\leq L_{\mathcal{N}}(\lambda)
-    \end{displaymath}
+        F_\mathcal{N}(\lambda)\leq L_{\mathcal{N}}(\lambda)$.
+    %\end{displaymath}
 \end{lemma}
-
 \begin{proof}
     We will prove that $\frac{1}{2}$ is a lower bound of the ratio $\partial_i
     F_\mathcal{N}(\lambda)/\partial_i L_\mathcal{N}(\lambda)$. Where
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