Degrees of Unsolvability in Formal Grammars. - Dennis F. Cudia; Wilson E. Singletary (J. ACM, 1968)

The Post Correspondence Problem. - Dennis F. Cudia; Wilson E. Singletary (J. Symb. Log., 1968)

The Degree Hierarchy of Undecidable Problems of Formal Grammars - Dennis F. Cudia (STOC, 1970)

General Problems of Formal Grammars. - Dennis F. Cudia (J. ACM, 1970)