Solomon Feferman. Degrees of unsolvability associated with classes of formalized theories. The journal of symbolic logic, vol. 22 (1957), pp. 161–175. - J. R. Shoenfield. Degrees of formal systems. The journal of symbolic logic, vol. 23 no. 4 (for 1958, pub. 1959), pp. 389–392.
Hartley Rogers · Journal of Symbolic Logic · 1962
Solomon Feferman. Degrees of unsolvability associated with classes of formalized theories. The journal of symbolic logic, vol. 22 (1957), pp. 161–175. - J. R. Shoenfield. Degrees of formal systems. The journal of symbolic logic, vol. 23 no. 4 (for 1958, pub. 1959), pp. 389–392. - Volume 27 Issue 1