Non-halting Turing machine

A non-halting Turing machine is a Turing machine that does not halt. These include halt-free Turing machines, meaning those without an undefined or halt transition, as well as non-halt-free Turing machines that never enter an undefined or halt transition.

The crux of the Busy Beaver problem, of finding BB(n, k) for a given n and k, is to prove that all non-halting Turing machines with n states and k symbols are, in fact, non-halting.

The zoology of non-halting Turing machines is extremely rich. See Translated cycler, Bouncer, Bell, Counter, Shift overflow counter, Shift overflow bouncer counter for a sample. In this page, we provide a detailed zoology for some low numbers of states and symbols.

Zoology

Machines are enumerated in TNF-1RB, and we exclude halting machines. In particular, a transition is defined if and only if it is reachable; unreachable transitions are undefined. This avoids duplicates.

2 × 2

There are 106 TNF-1RB machines with 2 states and 2 symbols, with the following breakdown:

Classification	Count	Notable examples
Translated cycler	88	1RB0RB_1LB1RA (bbch), unique TM with a record period of 9. 1RB0LB_1LA0RB (bbch), unique TM with a record preperiod of 9.
Cycler	14	1RB0RB_1LB1LA (bbch), one of the 7 TMs with a record period of 4. 1RB1LB_1LA1RA (bbch), unique TM with a record preperiod of 5.
Bouncer	3	1RB1LA_0LA1RB (bbch) 1RB1LA_1LA1RB (bbch) 1RB0LB_1LA0RA (bbch)
Counter	1	1RB1LA_0LA0RB (bbch), a binary counter.

3 × 2

There are 15,064 TNF-1RB machines with 3 states and 2 symbols, with the following breakdown:

Classification	Count	Notable examples
Translated cycler	12,427	1RB0LA_0RC1LA_1LC0RB (bbch), unique TM with a record period of 92. 1RB1LB_0RC0LA_1LC0LA (bbch), unique TM with a record preperiod of 101.
Cycler	1,969	1RB0LB_1LB1LC_0RC1RA (bbch), unique TM with a record period of 18. 1RB1RC_1LC0LB_1RA1LA (bbch), unique TM with a record preperiod of 22.
Bouncer	558	1RB0LC_1RC0RB_1LA1LC (bbch), a bouncer with two distinct shift rules.
Counter	95	1RB1LA_0LA0RC_0LA1RB (bbch), a Fibonacci counter. 1RB1LA_0LA1RC_0LA0RB (bbch), a two-phase binary counter. 1RB1LA_0LA1RC_1LB0RB (bbch), a translating binary counter.
Cubic bell	10	1RB1LA_0LA0RC_0LA1RC (bbch), a typical cubic bell.
Bell	5	1RB0LC_1RC1RA_1LA0RB (bbch), a typical bell. 1RB1LA_0RC0RC_1LC0LA (bbch), a typical inverted bell.

4 × 2

There are 2,744,516 TNF-1RB machines with 4 states and 2 symbols, with the following breakdown. This breakdown is not complete due the chaotic Turing machines. These are the irregular bells, spaghetti, and chaotic counters. These are informal names for chaotic behaviors which escape analysis, and such may well become a translated cycler, or more rarely, a bouncer, after a large number of steps.

Regular (non-chaotic)

Classification	Count	Notes and Notable examples	Example space-time diagram
Translated cycler	≥2,253,849	1RB0LA_0RC1RD_1LD0RB_1LA1RB (bbch), the current period record holder, with a period of 212,081,736. Before phasing into a translated cycle, this machine appears to be a spaghetti. 1RB1LC_0LA1RD_0RB0LC_1LA0RD (bbch), the current preperiod record holder, with a preperiod of 119,120,230,102. Before phasing into a translated cycle, this machine appears to be a spaghetti. 1RB1RC_1LC0RA_0LB0LD_1LA1LD (bbch) starts out as an irregular bell, but phase transitions into a translated cycler with period 4,222 at step 29,754,825.
Cycler	≥341,617	1RB0RB_1LC0RD_1LA1LB_0LC1RD (bbch), likely period record holder, with a period of 120. 1RB1LB_1LC1RD_1LA0RD_0LA0RB (bbch), likely preperiod record holder, with a preperiod of 146.
Bouncer	≈132,000	1RB1LA_1LC0RC_0RA1LD_1RC0LD (bbch), the current record holder for longest time to settle into a bouncer, with a start step of 83,158,409. 1RB0RC_0RC0RB_1LC0LD_1LA0RA (bbch), starts out as irregular-side bells before phasing into a bouncer at step 3350. 1RB1LC_0RD0LC_1LB0LA_1LD1RA (bbch), a bouncer with very complex runs. Start step 145,729.
Counter	≈14,700	1RB1LC_0LC1RD_1LA1LB_0LC0RD (bbch), a ternary counter. 1RB1LC_0LA1RB_1LD0RB_1LA1RA (bbch), a quaternary counter. 1RB0LB_1RC0LD_1LB1RA_0RB1LD (bbch), a quinary counter. 1RB1LC_0LD1RB_1LD0RD_1LA0RB (bbch), a senary counter. 1RB1LC_0LD0RB_1RD1LA_1RA0LC (bbch), a 3/2-counter. 1RB0RA_1LC1RA_1LD0LC_1LA1LD (bbch), a binary bi-counter. 1RB1LC_1RC1RB_1RD0LC_1LA0RD (bbch), a binary-ternary bi-counter. 1RB1LA_0LA0RC_0LA0RD_0LA1RC (bbch), a counter encoding a recurrence with characteristic polynomial $x^{3}-x^{2}-2x+1$. 1RB1LA_0LA0RC_0LA1RD_0LA0RB (bbch), a counter encoding a recurrence with characteristic polynomial $x^{3}-x^{2}-2$. 1RB1LC_1LD1RA_0RA0LC_0RB0LD (bbch), a counter encoding a recurrence that grows like $n\cdot 2^{n}$. 1RB0RC_0LC0RA_1LA0LD_1RA1LD (bbch), a tri-phasic binary counter. 1RB1LC_0RD0RB_1LA0LA_1LD0LA (bbch), an example of a superexponential counter.
Bell	≈2,350	1RB1LA_0RC0LD_1LC0LA_1RC0RD (bbch), a typical inverted bell. 1RB1LB_1LC0RA_1RD0LB_1LA1RC (bbch), alternates between bell and half-bell. 1RB0LC_1RC1RB_1LA1LD_0RA0RB (bbch), a grow-and-shrink bell. 1RB0RC_1LC0RA_1RA1LD_0LC0LA (bbch), starts out as an irregular bell before phasing into a bell.
Cubic bell	≈1,376	1RB1RC_1LC0RC_0RA1LD_0LC0LB (bbch), a cubic inverted bell. 1RB0RC_0LD0RA_0LA1RC_1LA1LD (bbch), a cubic grow-and-shrink bell.
Bouncer + X	≈365	1RB1LA_1RC0RB_0LC1LD_0LD1RA (bbch), a bouncer + binary counter. 1RB0LA_0RC1LA_1RD0RA_0LB1RB (bbch), a bouncer + bell. 1RB1LC_0RC1RD_1LA0LA_1RC0RB (bbch), a bouncer + cellular automaton. This could be universal. 1RB1LC_0RC1RD_1LA0LA_0LA0RB (bbch), a bouncer + cellular automaton with a fractal nature. 1RB1LB_0LC0RD_0RA1LC_1RA1RD (bbch), a bouncer + cubic bell, leading to quartic tape growth on the left. 1RB0LC_1LA1RD_1RA1LD_0LA0RB (bbch), a bouncer + unclassified. (If you can classify it, let me know in the talk page!)
Bounce-counter	≈330	1RB1LC_1LC0RB_1RA0LD_1RA1LA (bbch), a typical binary bounce-counter. 1RB1LB_1RC1RD_0LA0RC_1LD0LB (bbch), a typical quaternary bounce-counter. 1RB1RA_0RC0LC_1LA0LD_0RA1LC (bbch), a ternary bounce-counter, which is more rare. 1RB0LA_0RC1LA_0RD1RB_1LD1LA (bbch), a hybrid quaternary-octal bounce-counter. 1RB0LC_1RD0RB_1LA1RC_1LC1RB (bbch), a 3/2-bounce-counter. 1RB0LC_1LC0RD_1RA1LA_0RA0LA (bbch), a binary bounce-counter with stationary counter digits.
Fractal	20	1RB1RC_0RC0RB_0LD1LA_1LD0LA (bbch), a typical example.
Tetration counter	19	1RB1LC_0RD0RD_0RC0LA_1LD1RA (bbch), a typical example.
Cubic bounce-counter	13	1RB1RA_0LC0RB_0RA0LD_1LC1RD (bbch), a typical example. Note that these share many of the same properties as dekaheptoids.

Chaotic

Classification	Count	Notes and Notable examples	Example space-time diagram
Irregular bell	39	1RB0RC_1LC1RA_1RA1LD_0LC0LA (bbch), a typical irregular bell. 1RB1LA_0RC0RD_1LD1RC_1LD0LA (bbch), a typical irregular inverted bell.
Spaghetti	26	This is an informal description for spaghetti-code Turing machines that seem to have no predictable behavior, instead winding back and forth like a spaghetti. Any of these machines could potentially end up proven as one of the regular classifications. Indeed, many translated cyclers start their life out as spaghetti. 1RB0RB_1LC1RC_0RA1LD_1RC0LD (bbch), a typical spaghetti. 1RB0RA_1RC0RD_1LD1LC_1RA0LC (bbch), a spaghetti that seems to converge to a bounce-counter. 1RB1LC_0LA0RD_1LA0LB_1LA1RD (bbch), a spaghetti whose envelope seems to converge to that of a bouncer. 1RB0RC_1LC1LD_1RD1LB_1RA0LB (bbch), a cellular-automaton-like bouncer. The spaghetti nature of this machine is local. 1RB1LC_1LA1RD_1RA0LC_1LB0RD (bbch), a "spaghetti sandwich" -- a spaghetti sandwiched on the left and right by a growing predictable repeating pattern.
Chaotic counter	10	Chaotic counters have slow-growing tapes like counters, but the behavior seems to be chaotic and is as of yet unknown: 1RB0RC_0LD1RC_1LD0RB_0LA1LA (bbch) 1RB1RA_0LC0LD_1LD0RB_0RA1LC (bbch) 1RB0RC_1LA1RC_0LD0RB_0LA1LD (bbch) 1RB0RB_1LC1RA_1LD0LC_0RA0LD (bbch) 1RB1LA_0RC1RC_0LD0RB_0LA1LD (bbch) 1RB1LC_0LC0RD_0LA1LA_0RB1RD (bbch) 1RB0RB_0LC1RA_1LD1LC_0RA0LD (bbch) 1RB1LC_1LD0RB_1RA0LC_0RA0LD (bbch) 1RB1LC_0LA0RB_1RD0LC_1LA0RD (bbch) 1RB1LC_1RD0RB_1LA0LC_0LA0RD (bbch)

Records

Translated cycler preperiod

• 3x2: 101 (proven): 1RB1LB_0RC0LA_1LC0LA (bbch) (period = 24)
• 4x2: 119,120,230,102 (current champion): 1RB1LC_0LA1RD_0RB0LC_1LA0RD (bbch) (period = 966,716)
• 2x4: 293,225,660,896 (current champion): 1RB2LA0RA3LA_1LA1LB3RB1RA (bbch) (period = 483,328)

Translated cycler period

• 3x2: 92 (proven): 1RB0LA_0RC1LA_1LC0RB (bbch) (preperiod = 0)
• 4x2: 212,081,736 (current champion): 1RB0LA_0RC1RD_1LD0RB_1LA1RB (bbch) (preperiod = 5,248,647,886)
• 2x4: 33,209,131 (current champion): 1RB0RA3LB1RB_2LA0LB1RA2RB (bbch) (preperiod = 63,141,841)