In: Advanced Math
For X ≠ ∅, an action of G on X is transitive if and only if, given x and y in X, there is some g ∈ G such that y = gx.
Proof: Suppose the action is transitive, so there is one orbit. Given x in X, its orbit must
fill up X, so every element of X has the form gx for some g ∈ G.
Conversely, suppose that for each x and y in X we can write y = gx for some g ∈ G. Fix
x ∈ X. Since every y ∈ X has the form gx for some g, every y is in the orbit of x. Thus X has only one orbit.
Suppose the action is transitive, so there is one orbit. Given x in X, its orbit must
fill up X, so every element of X has the form gx for some g ∈ G.
Conversely,
suppose that for each x and y in X we can write y = gx for some g ∈ G. Fix x ∈ X. Since every y ∈ X has the form gx for some g, every y is in the orbit of x. Thus X has only one orbit.