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Diffstat (limited to 'paper/sections/model.tex')
| -rw-r--r-- | paper/sections/model.tex | 30 |
1 files changed, 6 insertions, 24 deletions
diff --git a/paper/sections/model.tex b/paper/sections/model.tex index 98712d7..25a0a47 100644 --- a/paper/sections/model.tex +++ b/paper/sections/model.tex @@ -26,16 +26,16 @@ problems. \subsection{Independent Cascade Model} -In the independent cascade model, nodes can be either uninfected, active or -infected. All nodes start either as uninfected or active. At each time step -$t$, for each edge $(i,j)$ where $j$ is uninfected and $i$ is active, $i$ +In the independent cascade model, nodes can be either susceptible, active or +infected. All nodes start either as susceptible or active. At each time step +$t$, for each edge $(i,j)$ where $j$ is susceptible and $i$ is active, $i$ attempts to infect $j$ with probability $p_{i,j}\in[0,1]$. If $i$ succeeds, $j$ will become active at time step $t+1$. Regardless of $i$'s success, node $i$ will be infected at time $t+1$: nodes stay active for only one time step. The cascade continues until no active nodes remain. If we denote by $X^t$ the indicator variable of the set of active nodes at time -step $t-1$, then if $j$ is uninfected at time step $t-1$, we have: +step $t-1$, then if $j$ is susceptible at time step $t-1$, we have: \begin{displaymath} \P\big[X^{t+1}_j = 1\,|\, X^{t}\big] = 1 - \prod_{i = 1}^m (1 - p_{i,j})^{X^t_i}. @@ -50,25 +50,7 @@ as: \end{equation} where we defined $\theta_j\defeq (\Theta_{1,j},\ldots,\Theta_{m,j})$. -\subsection{Linear Threshold Model} - -In the Linear Threshold Model, each node $j\in V$ has a threshold $t_j$ drawn -uniformly from the interval $[0,1]$. Furthermore, there is a weight -$\Theta_{i,j}\in[0,1]$ for each edge $(i,j)$. We assume that the weights are -such that for each node $j\in V$, $\sum_{i=1}^m \Theta_{i,j} \leq 1$. - -Nodes can be either uninfected or active. At each time step, each uninfected -node $j$ becomes active if the sum of the weights $\Theta_{i,j}$ for $i$ an -active parent of $j$ is larger than $j$'s threshold $t_j$. Denoting by $X^t$ -the indicator variable of the set of active nodes at time step $t-1$, if -$j\in V$ is uninfected at time step $t-1$, then: -\begin{equation} - \label{eq:lt} - \tag{LT} - \P\big[X^{t+1}_j = 1\,|\, X^{t}\big] = \sum_{i=1}^m \Theta_{i,j}X^t_i - = \inprod{\theta_j}{X^t} -\end{equation} -where we defined again $\theta_j\defeq (\Theta_{1,j},\ldots,\Theta_{m,j})$. +\subsection{Generalized Linear Models} \subsection{Maximum Likelihood Estimation} @@ -104,7 +86,7 @@ a separate optimization program: \end{equation} Furthermore, the state evolution of a node $j\in V$ has the same structure in -both models: the transition from uninfected to active at time step $t+1$ is +both models: the transition from susceptible to active at time step $t+1$ is controlled by a Bernoulli variable whose parameter can be written $f(\inprod{\theta_j}{x^t})$ for some function $f$. Hence, if we denote by $n_j$ the first step at which $j$ becomes active, we can rewrite the MLE program |
