In TD/SARSA-lambda, eligibility traces are decayed after each step by multiplying by the discount rate and the trace-decay parameter.
I understand that:
- The discount rate is used to reduce the value of future actions relative to a state.
- An eligibility trace should decay over time, as it represents how "responsible" a state-action pair is for an obtained reward.
What I don't understand why the trace-decay parameter is required, given that multiplying by the discount rate after each step will cause it to decay over time anyways. What does the trace-decay parameter add to the algorithm?