1. The word problem;Boone, William W.;Ann. of Math. (2),1959
2. McGraw-Hill Series in Information Processing and Computers;Davis, Martin,1958
3. Recursively enumerable sets of positive integers and their decision problems;Post, Emil L.;Bull. Amer. Math. Soc.,1944
4. A detailed argument for the Post-Linial theorems;Yntema, Mary Katherine;Notre Dame J. Formal Logic,1964