1.

Goldstone, J., Salam, A., Weinberg, S.: Phys. Rev.**127**, 965 (1962).

3.

Actually, the right-hand side of Eq. (2) is intolerably crudely defined. For a proper definition, see Kastler, D., Robinson, D., Swieca, A.: Commun. math. Phys.**3**, 151 (1966). The use of the proper definition does not affect the proof given in the text, except by making some of the equations look more complicated.

4.

As stated, for example. In: Streater, R., Wightman, A.: TCP, Spin and Statistics, and All That. New York: R. A. Benjamin 1964.

5.

This is the procedure of Kastler, Robinson, and Swieca (Ref. 3). The desired generalization has been proved by L. Landau (private communication).

6.

Mermin, N. D., Wagner, H.: Phys. Rev. Letters**17**, 1133 (1966).

7.

Englert, F., Brout, R.: Phys. Rev. Letters**13**, 321 (1964).

7a.

Higgs, P.: Phys. Letters**12**, 132 (1964).

7b.

Guralnik, G., Hagen, C., Kibble, T.: Phys. Rev. Letters**13**, 585 (1964).

7c.

Higgs, P.: Phys. Rev.**145**, 1156 (1966).

7d.

Kibble, T.: Phys. Rev.**155**, 1554 (1967).

9.

Private communication (through A. Wightman).

10.

This is an old observation. Schroer, B.: Fortschr. der Physik**11**, 1 (1963) and Wightman, A.: in High Energy Electromagnetic Interactions and Field Theory, ed. by Levy, M.: New York: Gordon and Breach 1967. This statement should not be taken to mean that there are no zero mass scalar particles in two dimensions. Indeed, if one defines “particle” in the usual way, as a normalizable eigenstate of*P*
_{μ}
*P*
^{μ}, the usual two-dimensional theory of massless Dirac fields contains massless scalar particles; these are states of one fermion and one antifermion, both in normalizable states moving to the left. It is a peculiarity of massless two-dimensional kinematics that, despite the fact that this is a normalizable two-particle state in Fock space, it is still an eigenstate of*P*
_{μ}
*P*
^{μ}. Consistent with the remarks above, though, the field :\(\bar \psi \psi\):, whose two-point function one might expect to possess a delta-function singularity because of the existence of these states, has in fact zero amplitude for creating these states from the vacuum.