two block probability

Figure 1.29: Two-block probabilities, showing a linear (rather than quadratic) relation valid at low densities, slope 1/2, holding over an extensive range.

In the contour map of pair probabilities, shown in Figure 1.29, single 1's, whose frequency forms the horizontal coordinate, tend to occur with double the frequency of pairs of 1's, whose frequency defines the vertical coordinate. That would indicate that the 1's are more likely to form blocks than to be isolated, a predilection compatible with the scarcity of sequences in which 0's and 1's alternate.