Reading assignment: sections 9.1, 9.2 of text. Recommended reading: sections 9.5, 9.6 of text. Turing Machines. Basic definitions. abnormal termination; note that the text's use of the term "halt" excludes abnormal termination; Turing Machines as language acceptors: A subset F of the TM states is identified as "final" states; A string u is "accepted" by M if M(u) halts at a final state; A language L that is accepted by a Turing Machine M is said to be "recursively enumerable" (alternatively, an "re set"). It is important to note that M need not halt for strings not in L. Thus, this notion of acceptance is asymmetric: M halts in an accepting stace for strings in L, but for strings not in L, M may loop. This has important implications. In particular, a string not in L is never really rejected by M. A Turing Machine that halts on all inputs is said to be a "halting" machine. A language L which is accepted by a halting TM is said to be "recursive". Question: does there exist an re set which is not recursive? We showed a TM that accepts L = {a n b n c n | n>= 0}.