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How frequently is a system of 2-linear Boolean equations solvable?

Combinatorics 2010-05-13 v1 Discrete Mathematics

Abstract

We consider a random system of equations xi+xj=b(i,j)(mod 2)x_i+x_j=b_{(i,j)} (\text{mod }2), (xu{0,1},b(u,v)=b(v,u){0,1})(x_u\in \{0,1\},\, b_{(u,v)}=b_{(v,u)}\in\{0,1\}), with the pairs (i,j)(i,j) from EE, a symmetric subset of [n]×[n][n]\times [n]. EE is chosen uniformly at random among all such subsets of a given cardinality mm; alternatively (i,j)E(i,j)\in E with a given probability pp, independently of all other pairs. Also, given EE, \pr{be=0}=\pr{be=1}\pr\{b_{e}=0\}=\pr\{b_e=1\} for each eEe\in E, independently of all other beb_{e^\prime}. It is well known that, as mm passes through n/2n/2 (pp passes through 1/n1/n, resp.), the underlying random graph G(n,#edges=m)G(n,\#\text{edges}=m), (G(n,\pr(edge)=p)G(n,\pr(\text{edge})=p), resp.) undergoes a rapid transition, from essentially a forest of many small trees to a graph with one large, multicyclic, component in a sea of small tree components. We should expect then that the solvability probability decreases precipitously in the vicinity of mn/2m\sim n/2 (p1/np\sim 1/n), and indeed this probability is of order (12m/n)1/4(1-2m/n)^{1/4}, for m<n/2m<n/2 ((1pn)1/4(1-pn)^{1/4}, for p<1/np<1/n, resp.). We show that in a near-critical phase m=(n/2)(1+\lan1/3)m=(n/2)(1+\la n^{-1/3}) (p=(1+\lan1/3)/np=(1+\la n^{-1/3})/n, resp.), \la=o(n1/12)\la=o(n^{1/12}), the system is solvable with probability asymptotic to c(\la)n1/12c(\la)n^{-1/12}, for some explicit function c(\la)>0c(\la)>0. Mike Molloy noticed that the Boolean system with be1b_e\equiv 1 is solvable iff the underlying graph is 22-colorable, and asked whether this connection might be used to determine an order of probability of 22-colorability in the near-critical case. We answer Mike's question affirmatively and show that probability of 22-colorability is 21/4e1/8c(λ)n1/12\lesssim 2^{-1/4}e^{1/8}c(\lambda)n^{-1/12}, and asymptotic to 21/4e1/8c(\la)n1/122^{-1/4}e^{1/8}c(\la)n^{-1/12} at a critical phase \la=O(1)\la=O(1), and for \la\la\to -\infty. (Submitted to Electronic Journal of Combinatorics on September 7, 2009.)

Cite

@article{arxiv.1005.1951,
  title  = {How frequently is a system of 2-linear Boolean equations solvable?},
  author = {Boris Pittel and Ji-A Yeum},
  journal= {arXiv preprint arXiv:1005.1951},
  year   = {2010}
}
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