Dear all！

One part of my gams model is to calculate the volume of equipment retired in year t and I used the following equation:

```
v_outflow(t) .. out(t) =e= sum((tp,p)$(ord(tp) Lt ord(t) and ord(p) eq ord(t)- ord(tp)),in(tp) * out_factor(p));
```

Basic idea of the equation is that the volume of equipment retired in year_t_ ,v_outflow(t,i), is composed of equipment produced from year_1_ to year_tp_( tp <= t ) and the retire proportion of equipment produced in year t when it has been used for p years(p = t – tp) is out_factor(p). and the in(tp) means the equipment produced in year_tp_.

The gamside tells me that the syntax of the sentence is correct, while I can’t get the correct answer based on the formula. So can anyone help me with identifying the error of the formula? Or is there any better method to compute the retired volume.