1. S. D. Drell and T. D. Lee, Phys. Rev. D 5, 1738 (1972). The fields P(x) and X(x) used in this reference correspond, respectively, to ψ(x) and π(x) in the present paper. See also the various other references mentioned therein.
2. Formally, one may consider the Lagrangian of this well-defined theory to be of the form L0-ηπ4, where L0 is given by (7) and η is a function of k0 satisfying lim
$$lim_{k_0 \rightarrow 0} k_0^n \eta \rightarrow 0$$
for all negative powers n. As an example, one may choose, say, η = exp(−b/k0
2) where b is a positive constant. The corresponding Hamiltonian now clearly has a lower bound. By choosing the appropriate renormalization counterterms, one can easily arrange in this theory to have an (asymptotic) expansion in powers of k0 which is determined by L0 only.
3. K. Johnson, Nucl. Phys. 25, 435 (1961).
4. R. P. Feynman, Phys. Rev. Letters 23, 1415 (1969); in High Energy Collisions, Third International Conference held at the State University of New York, Stony Brook, 1969, edited by C. N. Yang et al. (Gordon and Breach, New York, 1969)
5. J. D. Bjorken and E. A. Paschos, Phys. Rev. 185, 1975 (1969).