Library

ITM · Recursively enumerable

Two cells, forever

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

It has a halt — state A reading a 1 — and never reaches it: after two steps it bounces between two cells, and its whole configuration repeats every 2 steps. Proof: a cycler.

Open in AutomataStudio
More
Open in the desktop app Download the .automaton file Source on GitHub
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.

Never halts from a blank tape. Proven: a configuration repeats exactly (a cycler), period 2.

Standard format0LB1RZ_1RA1RA_1RC1RBView on bbchallenge.org ↗

Notes

Standard format: 0LB1RZ_1RA1RA_1RC1RB