Following the example of the 1st equation, why doesn't the 2nd and 3rd equation result in
$$\sum_{all\,x} \,\, \sum_{y:y=x} P(x,y) $$
$$\sum_{all\,x} \,\, \sum_{y:y=z-x} P(x,y) $$ ?
Following the example of the 1st equation, why doesn't the 2nd and 3rd equation result in
$$\sum_{all\,x} \,\, \sum_{y:y=x} P(x,y) $$
$$\sum_{all\,x} \,\, \sum_{y:y=z-x} P(x,y) $$ ?
It does. However $$\sum_{y\colon y=z}f(y)=f(z).$$