https://en.wikipedia.org/wiki/Element_(mathematics)#Notation_and_terminology https://en.wikipedia.org/wiki/Element_(mathematics)#Notation... I'm not sure what

https://en.wikipedia.org/wiki/Element_(mathematics)#Notation_and_terminology https://en.wikipedia.org/wiki/Element_(mathematics)#Notation... I'm not sure what the third line is supposed to look like, but I guess it's a filter. So: Sum i^2 where i is in the set {1,2,3,4,5} and i % 3 = 1.