  We study a reversible one-dimensional spin system with Bernoulli(p)
stationary distribution, in which a site can flip only if the site to its left
is in state +1. Such models have been used as simple exemplars of systems
exhibiting slow relaxation. We give fairly sharp estimates of the spectral gap
as p decreases to zero. The method uses Poincare comparison with a long-range
process which is analyzed by probabilistic methods (coupling,
supermartingales).
