# Clarifications on “Prioritized Experience Replay” (Deepmind, 2015)

Paper link : Prioritized Experience Replay

1. Why is the number of possible action sequences equal to 2^N? I cant think of sequences more than (N + 1) where one sequence is the sequence of all right actions and the other N sequences are due to wrong actions at each state.

Generally for prioritized experience replay:

1. The replay memory consists of some transitions which are repeated.In the priority queue I feel that there should only be a single priority for each transition to speed up learning. Is there any advantage of having priority values for each repeated instance of the transition?

Edit for 2nd question:

Consider algorithm 1 on page 5 of the article.

Lets consider one of the transitions to be repeated in the replay memory. If one of them is sampled (line 9) and the priority updated (line 12). Will the priority update on the other instance of the same transition?

for 1) i think your confusing elements touched vs sequences. At each point for N turns you have 2 possible options, therefore you have $$\prod_{i=1}^N 2$$ or $$2^N$$ possible sequences.