With the usual conventions of defining and , these functions map the set onto itself. In geometric language, they are projectivities of the projective line. They form a group under composition which is the projective special linear group (2,11) of order 660. There are exactly five elements of this group that leave the starting block fixed setwise, namely those such that and so that . So there will be 660/5 = 132 images of that block. As a consequence of the multiply transitive property of this group acting on this set, any subset of five elements of will appear in exactly one of these 132 images of size six.