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derivative decreases in a linear fashion with increasing β , except for a small roundoff region that we interpret as a finite-size effect. We then estimate β$_{c}$ as the point where dJ/dβ falls to zero, using the data in the linear region. The procedure for fixed β is analogous. The continuous transition points obtained by this procedure are quite robust with respect to changes in the region analyzed, as long as we exclude the roundoff region. 

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Using the methods described above we construct a phase diagram based on the data for each system size studied; that for L = 500 is shown in Fig. 9. The vertical boundary at α = 0 . 905(2), and the horizontal boundary at β = 0 . 714(1) are obtained using the derivative method. (The polynomial method yields α = 0 . 8(1) and β = 0 . 67(5), respectively, for these boundaries.) The grey triangle in Fig. 9 represents a region on which we could not determine precisely the phase boundaries using either method, due to numerical uncertainty and finite-size effects. While the simplest interpretation is that the three phase boundaries meet at the point of intersection between the continuous transitions (i.e., horizontal and vertical) lines, the data in 

