Automata theory and formal language theory are closely related to computability. Computable functions are the basic objects of study in computability theory. Recursion theory also includes the study of generalized computability and definability. It developed into a study of abstract computability, which became known as recursion theory. In the late 19th century, Leopold Kronecker formulated notions of computability, defining primitive recursive functions. Computability theory for digital computation is well developed. The fields of feasible computability and computational complexity study functions that can be computed efficiently. For other computational and complexity subjects, see list of computability and complexity topics. Mihara shows that such a rule violates algorithmic computability. Closely related fields in theoretical computer science are analysis of algorithms and computability theory. Another way of stating Rice's theorem that is more useful in computability theory follows. This constituted a plausibility result in computability theory. Therefore, formal language theory is a major application area of computability theory and complexity theory. The Busy Beaver Game has implications in computability theory, the halting problem, and complexity theory. While there Kenneth researched in group theory and computability theory.