[1] On May 14, 2007, Wolfram announced a $25,000 prize to be won by the first person to prove or disprove the universality of the (2,3) Turing machine.
Since the proof applies to a non-standard Turing machine model which allows infinite, non-periodic initial configurations, it is categorized by some as "weak-universal".
[3] Claude Shannon first explicitly posed the question of finding the smallest possible universal Turing machine in 1956.
The table entries indicate the symbol to be printed, the direction in which the tape head is to move, and the subsequent state of the machine.
Therefore, though it may be true that the (2,3) machine is in some sense the "smallest possible universal Turing machine", this has not been strictly proven in the classical sense, and the claim is open to debate when taking into consideration traditional definitions of universality and whether the relaxing of the Turing machine properties used for the proof can be allowed in general and may even suggest novel ways to define computational universality more independent of arbitrary choices (such as having a halting configuration or requiring a blank tape).
Smith first constructed a sequence of rule systems showing that the (2,3) Turing machine is capable of arbitrary finite computations.
The announcement that Alex Smith's proof had won was made without the approval of the judging committee,[20] as noted by Martin Davis, a member of the committee, in a post to the FOM mailing list: Vaughan Pratt subsequently disputed the correctness of this proof in a post to the mailing list,[22] noting that similar techniques would allow a linear bounded automaton (or LBA) to be universal, which would contradict a known non-universality result due to Noam Chomsky.