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In page Busy beaver:

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The longest running 6-state, 2-symbol machine which has the additional property of reversing the tape value at each step produces 6147 1s after 47339970 steps.[citation needed] So for the Reversal Turing Machine (RTM) class,[2] SRTM(6) ≥ 47339970 and ΣRTM(6) ≥ 6147. Likewise we could define an analog to the Σ function for register machines as the largest number which can be present in any register on halting, for a given number of instructions.[citation needed]