RMTboy

If I understand well, the quantity to optimise is the credit $C$ obtained which is expressed as follows. If the number of PMRQ is $N$ then the total cost is

$\sum_{t=1}^T \sum_{i=1}^{N}cost_i PMRQ_{i,t}\times (1-\sum_{j=1}^{N} credit_{i,j}PMRQ_{j,t})$ and the credit obtained is

where $PMRQ_{i,j}$ is 1 is the maintenance of $i$ is done at time $t$ and 0 otherwise. The variable $credit(i,j)$ indicates whether the maintenance of PMRQ-i is free if we do the one of PMRQ-j.

I guess there is a small typo in the sentence "PMRQ 1 credit PMRQ 2 and 4 for a cost of 5 at period 5" where it should be a cost of 4 and not 5. We'll rewrite the description of the problem to make it a bit more friendly!