  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.
