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ITM · Recursively enumerable

BB(3) champion — most steps

@thethinkmachine·version 1·CC BY 4.0·added 28 Sept 2026

Three states, 21 steps: the longest-running halting 3-state machine.

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ABChalt
The machine as drawn — 4 states, 6 transitions.

Behaviour

The first steps from a blank tape: one row per step, time running down; the outlined cell is the head.

Halts from a blank tape after 21 steps, leaving 5 non-blank cells.

Standard format1RB1RZ_1LB0RC_1LC1LAView on bbchallenge.org ↗

Notes

Standard format: 1RB1RZ_1LB0RC_1LC1LA