Let $f(x)$ be the set of all even numbers smaller or equal than $x$.
For example, $f(10)=\{0,2,4,6,8,10\}$. Let $g(x)=2x$.
Consider the double sum: $\sum_{i=1}^4 \sum_{j\in(g \circ f)(i)} j$
I know that $(g \circ f)(i)=g(f(i))$
Where should the summation start from?