- Apply a to time series data.
- Use output of network unites at time t as input to units at time t+1.
- They need a new training algorithm. Several have been proposed.
- A common method of training is to use a simple variation on backpropagation.
- This method involves "unfolding" the network over time (as seen in (c)) and then using then applying backpropagation to the new network.