The Number of Independent Sets in a Grid Graph

  • Authors:
  • Neil J. Calkin;Herbert S. Wilf

  • Affiliations:
  • -;-

  • Venue:
  • SIAM Journal on Discrete Mathematics
  • Year:
  • 1998

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Abstract

If f(m,n) is the (vertex) independence number of the $m\times n$ grid graph, then we show that the double limit $\eta\eqdef\lim_{m,n\to\infty}f(m,n)^{1\over {mn}}$ exists, thereby refining earlier results of Weber [Rostock. Math. Kolloq., 34 (1988), pp. 28--36] and Engel [Fibonacci Quart.,, 28 (1990), pp. 72--78]. We establish upper and lower bounds for $\eta$ and {\it prove} that $1.503047782... \le \eta \le 1.5035148\ldots $. Numerical computations suggest that the true value of $\eta$ (the "hard square constant") is around 1.5030480824753323... .