Convergence of the Linear Delta Expansion in the Critical O(N) Field   Theory
  The linear delta expansion is applied to the 3-dimensional O(N) scalar field theory at its critical point in a way that is compatible with the large-N limit. For a range of the arbitrary mass parameter, the linear delta expansion for <phi^2> converges, with errors decreasing like a power of the order n in delta. If the principal of minimal sensitivity is used to optimize the convergence rate, the errors seem to decrease exponentially with n. 
Eric Braaten and Eugeniu Radescu