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Diffstat (limited to 'paper/sections/model.tex')
| -rw-r--r-- | paper/sections/model.tex | 43 |
1 files changed, 20 insertions, 23 deletions
diff --git a/paper/sections/model.tex b/paper/sections/model.tex index 532ee5e..5d3a232 100644 --- a/paper/sections/model.tex +++ b/paper/sections/model.tex @@ -130,14 +130,12 @@ Defining $\Theta_{i,j} \defeq \log(\frac{1}{1-p_{i,j}})$, this can be rewritten \end{align*} Therefore, the independent cascade model is a Generalized Linear Cascade model -with inverse link function $f : z \mapsto 1 - e^{-z}$. - -Note that to write the Independent Cascade Model as a Generalized Linear -Cascade Model, we had to introduce the change of variable $\Theta_{i,j} -= \log(\frac{1}{1-p_{i,j}})$. The recovery results in Section~\ref{sec:results} -pertain to the $\Theta_j$ parameters. Fortunately, the following lemma shows that -the recovery error on $\Theta_j$ is an upper bound on the error on the original -$p_j$ parameters. +with inverse link function $f : z \mapsto 1 - e^{-z}$. Note that to write the +Independent Cascade Model as a Generalized Linear Cascade Model, we had to +introduce the change of variable $\Theta_{i,j} = \log(\frac{1}{1-p_{i,j}})$. +The recovery results in Section~\ref{sec:results} pertain to the $\Theta_j$ +parameters. Fortunately, the following lemma shows that the recovery error on +$\Theta_j$ is an upper bound on the error on the original $p_j$ parameters. \begin{lemma} \label{lem:transform} @@ -181,12 +179,12 @@ Let $\text{Exp}(p)$ be an exponentially-distributed random variable of parameter and let $\Theta_{i,j}$ be the rate of transmission along directed edge $(i,j)$ in the CICE model. By the memoryless property of the exponential, if $X^k_j \neq 1$: -\begin{align*} - \mathbb{P}(X^{k+1}_j = 1 | X^k) & = \mathbb{P}(\min_{i \in {\cal N}(j)} +\begin{multline*} + \mathbb{P}(X^{k+1}_j = 1 | X^k) = \mathbb{P}(\min_{i \in {\cal N}(j)} \text{Exp}(\Theta_{i,j}) \leq \epsilon) \\ - & = \mathbb{P}(\text{Exp}( \sum_{i=1}^m \Theta_{i,j} X^t_i) \leq \epsilon) \\ - & = 1 - e^{- \epsilon \Theta_j \cdot X^t} -\end{align*} + = \mathbb{P}(\text{Exp}( \sum_{i=1}^m \Theta_{i,j} X^t_i) \leq \epsilon) + = 1 - e^{- \epsilon \Theta_j \cdot X^t} +\end{multline*} Therefore, the $\epsilon$-discretized CICE-induced process is a Generalized Linear Cascade model with inverse link function $f:z\mapsto 1-e^{-\epsilon\cdot z}$. @@ -194,10 +192,8 @@ Generalized Linear Cascade model with inverse link function $f:z\mapsto \subsubsection{Logistic Cascades} \label{sec:logistic_cascades} ``Logistic cascades'' is the specific case where the inverse link function is -given by the logistic function: -\begin{displaymath} - f(z) = \frac{1}{1+e^{-z + t}} -\end{displaymath} +given by the logistic function +$f(z) = 1/(1+e^{-z + t})$. Intuitively, this captures the idea that there is a threshold $t$ such that when the sum of the parameters of the infected parents of a node is larger than the threshold, the probability of getting infected is close to one. This is @@ -277,11 +273,11 @@ and: \end{multline*} In the case of the voter model, the measurements include all time steps until -we reach the time horizon $T$ or the graph coalesces to a single state. In the -case of the independent cascade model, the measurements include all time steps -until node $i$ becomes contagious, after which its behavior is deterministic. -Contrary to prior work, our results depend on the number of -measurements and not the number of cascades. +we reach the time horizon $T$ or the graph coalesces to a single state. For the +independent cascade model, the measurements include all time steps until node +$i$ becomes contagious, after which its behavior is deterministic. Contrary to +prior work, our results depend on the number of measurements and not the number +of cascades. \paragraph{Regularity assumptions} @@ -294,12 +290,13 @@ Model and Voter Model. Furthermore, the data-dependent bounds in Section~\ref{sec:main_theorem} will require the following regularity assumption on the inverse link function $f$: +there exists $\alpha\in(0,1)$ such that \begin{equation} \tag{LF} \max \big\{ | (\log f)'(z_x) |, |(\log (1-f))'(z_x) | \big\} \leq \frac{1}{\alpha} \end{equation} -for some $\alpha\in(0, 1)$ and for all $z_x\defeq\inprod{\theta^*}{x}$ such that +for all $z_x\defeq\inprod{\theta^*}{x}$ such that $f(z_x)\notin\{0,1\}$. In the voter model, $\frac{f'(z)}{f(z)} = \frac{1}{z}$ and |
