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  EurusPRM-Stage2 is trained using **[Implicit PRM](https://arxiv.org/abs/2412.01981)**, which obtains free process rewards at no additional cost but just needs to simply train an ORM on the cheaper response-level labels. During inference, implicit process rewards are obtained by forward passing and calculating the log-likelihood ratio on each step.
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- <img src="./figs/implicit.png" alt="prm" style="zoom: 33%;" />
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  The key ingredient of Implicit PRM is the reward representation, as demonstrated below:
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  <aside>
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- ***Proposition***: Consider an ORM where the reward is parameterized by the log-likelihood ratio of two causal LMs, i.e. $r_\phi(\mathbf{y}):= \beta \log \frac{\pi_\phi(\mathbf{y})}{\pi_\text{ref}(\mathbf{y})}$. Define $q_\phi^t(\mathbf{y}_{<t}, y_t):= \sum_{i=1}^{t} \beta \log \frac{\pi_\phi(y_{i}|\mathbf{y}_{<i})}{\pi_\text{ref}(y_{i}|\mathbf{y}_{<i})}$. $q_\theta^t$ is the exponential average of $r_\theta$ at step $t$.*
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  $$
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  q_\phi^t(\mathbf{y}_{<t}, y_t) = \beta \log \mathbb{E}_{\pi_\text{ref}(\mathbf{y}|\mathbf{y}_{\leq t})} e^{\frac{1}{\beta}r_\phi(\mathbf{y})}
 
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  EurusPRM-Stage2 is trained using **[Implicit PRM](https://arxiv.org/abs/2412.01981)**, which obtains free process rewards at no additional cost but just needs to simply train an ORM on the cheaper response-level labels. During inference, implicit process rewards are obtained by forward passing and calculating the log-likelihood ratio on each step.
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+ <img src="./figures/implicit.png" alt="prm" style="zoom: 33%;" />
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  The key ingredient of Implicit PRM is the reward representation, as demonstrated below:
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  <aside>
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+ ***Proposition**: Consider an ORM where the reward is parameterized by the log-likelihood ratio of two causal LMs, i.e. $r_\phi(\mathbf{y}):= \beta \log \frac{\pi_\phi(\mathbf{y})}{\pi_\text{ref}(\mathbf{y})}$. Define $q_\phi^t(\mathbf{y}_{<t}, y_t):= \sum_{i=1}^{t} \beta \log \frac{\pi_\phi(y_{i}|\mathbf{y}_{<i})}{\pi_\text{ref}(y_{i}|\mathbf{y}_{<i})}$. $q_\theta^t$ is the exponential average of $r_\theta$ at step $t$.*
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  $$
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  q_\phi^t(\mathbf{y}_{<t}, y_t) = \beta \log \mathbb{E}_{\pi_\text{ref}(\mathbf{y}|\mathbf{y}_{\leq t})} e^{\frac{1}{\beta}r_\phi(\mathbf{y})}