  As noted, the primary reference for this work is Church 1951, but there are several other significant papers including Church 1973, Church 1974, and the Introduction to Church 1944, which contains an informal discussion of some of the ideas. In addition, the expository papers of Anderson are enlightening (Anderson 1984, 1998). It should be noted that there are relationships between Church’s work and that of Carnap, discussed below. Church’s ideas first appeared in an abstract (Church 1946), then Carnap’s book appeared (Carnap 1947). A few years later Church’s paper expanded his abstract in Church 1951. The second edition of Carnap’s book appeared in 1956. Each man had an influence on the other, and the references between the two authors are thoroughly intertwined.