Karp, Carol (August 1967). "S. Feferman and R. L. Vaught. The first order properties of products of algebraic systems. Fundamenta mathematicae, vol, 47 (1959), pp. 57–103. (Review)". Journal of Symbolic Logic. 32 (2): 276–276. doi:10.2307/2271704.
Mostowski, Andrzej (March 1952). "On direct products of theories". Journal of Symbolic Logic. 17 (1): 1–31. doi:10.2307/2267454.