Local Minima in Disordered Mean-Field Ferromagnets

Abstract

We consider the complexity of random ferromagnetic landscapes on the hypercube $$\{\pm \,1\}^N$$ given by Ising models on the complete graph with i.i.d. non-negative edge-weights. This includes, in particular, the case of Bernoulli disorder corresponding to the Ising model on a dense random graph $$\mathcal G(N,p)$$ . Previous results had shown that, with high probability as $$N\rightarrow \infty $$ , the gradient search (energy-lowering) algorithm, initialized uniformly at random, converges to one of the homogeneous global minima (all-plus or all-minus). Here, we devise two modified algorithms tailored to explore the landscape at near-zero magnetizations (where the effect of the ferromagnetic drift is minimized). With these, we numerically verify the landscape complexity of random ferromagnets, finding a diverging number of (1-spin-flip-stable) local minima as $$N\rightarrow \infty $$ . We then investigate some of the properties of these local minima (e.g., typical energy and magnetization) and compare to the situation where the edge-weights are drawn from a heavy-tailed distribution.