Define a relation R on Zopf by aRb iff 2 divides a + b. Find all equivalence classes w.r.t. R. Solution R: a congruent to -b ( mod 2 ) is an equivalence relation. We can easily see that it is reflexive,symmetric and transitive. a congruent to -b ( mod 2 ) iff either both a and b are odd or both a and b are even. The equivalence classes of R are [0]: all even integers [1]: all odd integers.