On the Birman-Schwinger principle applied to (-Delta + m^2)^(1/2) - m
  The condition for E = 0 to be an eigenvalue of the operator (-Delta + m^2)^(1/2) -m + l V is obtained through the use of the Birman-Schwinger principle. By setting E=-a^2 and using the analyticity of the corresponding Birman-Schwinger kernel, the series development of (l^(-1))(a) is obtained up to second order on a. 
Marco Maceda