Intersection

For the given arrays of integers a1 and a2, find all the items that belong to both a1 and a2.

E.g. for the following input:

var a1 = new[] { 1, 5, 10, 8 };
var a2 = new[] { 6, 8, 17, 4, 1 };

The output should be: 1, 8