Question

What is the meaning of {x} + {y}

Answer 1

List<int> list = new List<int> { 2, 5, 7, 10 }; int number = 9; int closest = list.Aggregate((x,y) => Math.Abs(x-number) < Math.Abs(y-number) ? x : y);

Answer 2

Are you talking about modulus functions?
Such as |x|?