Hello.
I thank you for your answer. It's very near to what i wanted. But the operation is not what I meant.
Let us consider your exemple, Every "lagged" value must be exactly the mpg + the half of the previous "lagged".
The first lagged should be 21
The second one should be 21 added to half of the first 21 = 31.5
The third one should be 22.8 added to half of 31.5 = 36.75
The fourth one should be 21.4 added to half of 36.75 = 39.775
And so on