Introduction | Bibliography

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:G4 receiver An alternate Herschel receiver discovered by Sergei Petrov on 28 December 2011, using his previous glider to 2 blocks converter. In the pattern below the Herschel output is marked by a ghost Herschel. A glider also escapes to the northwest. For an explanation of the "G4" describing the tandem glider input, see Gn.

    ................................O.O.......................
    ................................OO........................
    .................................O........................
    ..........................................................
    ..........................................................
    ..........................................................
    ..........................................................
    ................OO........................................
    ................OO....................O...................
    ...........................O.O......OOO...................
    ...........................OO......O......................
    ............................O......OO.....................
    ..........................................................
    ..........................................................
    ..........................................................
    OO........................................................
    OO........................................................
    ......................................OO...............O..
    ...........OO.........................OO...............O..
    ...........OO..........................................OOO
    .......................OO................................O
    ........O..........OO..OO.................................
    ......OOO..........OO.....................................
    .....O....................................................
    .....OO........................OO.........................
    .................OO............O..........................
    .................OO......O......OOO.......................
    ........................O.O.......O.......................
    .........................O................................

:Gabriel's p138 (p138) The following oscillator found by Gabriel Nivasch in October 2002.

    .......OOO.....
    ......O..O.....
    .......O...O...
    ..O.....OOO....
    ...O.....O.....
    OO.OO..........
    O..O.........O.
    O.O.........O.O
    .O.........O..O
    ..........OO.OO
    .....O.....O...
    ....OOO.....O..
    ...O...O.......
    .....O..O......
    .....OOO.......

:galaxy = Kok's galaxy

:Game of Life = Life

:Game of Life News A blog reporting on new Life discoveries, started by Heinrich Koenig in December 2004, currently found at http://pentadecathlon.com/lifenews/.

:Garden of Eden A configuration of ON and OFF cells that can only occur in generation 0. (This term was first used in connection with cellular automata by John W. Tukey, many years before Life.) It was known from the start that there are Gardens of Eden in Life, because of a theorem by Edward Moore that guarantees their existence in a wide class of cellular automata. Explicit examples have since been constructed, the first by Roger Banks, et al. at MIT in 1971. This example was 9 x 33. In 1974 J. Hardouin-Duparc et al. at the University of Bordeaux 1 produced a 6 x 122 example. The following shows a 12 x 12 example found by Nicolay Beluchenko in February 2006, based on a 13 x 12 one found by Achim Flammenkamp in June 2004.

    ..O.OOO.....
    OO.O.OOOOO.O
    O.O.OO.O.O..
    .OOOO.O.OOO.
    O.O.OO.OOO.O
    .OOO.OO.O.O.
    ..O...OOO..O
    .O.OO.O.O.O.
    OOO.OOOO.O.O
    OO.OOOO...O.
    .O.O.OO..O..
    .OO.O..OO.O.

Below is a 10x10 Garden of Eden found by Marijn Heule, Christiaan Hartman, Kees Kwekkeboom, and Alain Noels in 2013 using SAT-solver techniques. An exhaustive search of 90-degree rotationally symmetric 10x10 patterns was possible because the symmetry reduces the number of unknown cells by a factor of four.

    .O.OOO.O..
    ..O.O.O..O
    O.OOO..OO.
    .O.OOOOO.O
    O..O..OOOO
    OOOO..O..O
    O.OOOOO.O.
    .OO..OOO.O
    O..O.O.O..
    ..O.OOO.O.

Steven Eker has since found several asymmetrical Gardens of Eden that are slightly smaller than this in terms of bounding box area. Patterns have also been found that have only Garden of Eden parents. For related results see grandparent.

:Gemini ((5120,1024)c/33699586 obliquely, p33699586) The first self-constructing spaceship, and also the first oblique spaceship. It was made public by Andrew Wade on 18 May 2010. It was the thirteenth explicitly constructed spaceship velocity in Life, and made possible an infinite family of related velocities. The Gemini spaceship derives its name from the Latin "gemini", meaning twins, describing its two identical halves, each of which contains three Chapman-Greene construction arms. A tape of gliders continually relays between the two halves, instructing each to delete its parent and construct a daughter configuration.

:Gemini puffer See Pianola breeder.

:Geminoid A type of self-constructing circuitry that borrows key ideas from Andrew Wade's Gemini spaceship, but with several simplifications. The main feature common to the Gemini spaceship is the construction recipe encoding method. Information is stored directly, and much more efficiently, in the timings of moving gliders, rather than in a static tape with 1s and 0s encoded by the presence of small stationary objects.

Unlike the original Gemini, Geminoids have ambidextrous construction arms, initially using glider pairs on two lanes separated by 9hd, 10hd, or 0hd. The design was the basis for the linear propagator and the Demonoids. A more recent development is a Geminoid toolkit using a single-channel construction arm, which allows for the possibility of multiple elbows with no loss of efficiency, or the construction of temporary lossless elbows. Compare slow elbow.

Other new developments that could be considered part of the extended "Geminoid" toolkit include freeze-dried construction salvos and seeds, used when objects must be built within a short time window, and self-destruct circuits, which are used as an alternative to a destructor arm to clean up temporary objects in a similarly short window.

:generation The fundamental unit of time. The starting pattern is generation 0.

:germ (p3) Found by Dave Buckingham, September 1972.

    ....OO....
    .....O....
    ...O......
    ..O.OOOO..
    ..O....O..
    .OO.O.....
    ..O.O.OOOO
    O.O.O....O
    OO...OOO..
    .......OO.

:gfind A program by David Eppstein which uses de Bruijn graphs to search for new spaceships. It was with gfind that Eppstein found the weekender, and Paul Tooke later used it to find the dragon. It is available at http://www.ics.uci.edu/~eppstein/ca/gfind.c (C source code only).

Compare lifesrc.

:ghost Herschel A dying spark made by removing one cell from the Herschel heptomino. This particular spark has the advantage that, when placed in a conduit to mark the location of an input or output Herschel, it disappears cleanly without damaging adjacent catalysts, even in dependent conduits with a block only two cells away.

    O..
    O..
    OOO
    ..O

:GIG A glider injection gate. This is a device for injecting a glider into a glider stream. The injected glider is synthesized from one or more incoming spaceships assisted by the presence of the GIG. (This contrasts with some other glider injection reactions which do not require a GIG, as in inject.) Gliders already in the glider stream pass through the GIG without interfering with it. A GIG usually consists of a small number of oscillators.

For example, in July 1996 Dieter Leithner found the following reaction which allows the construction of a pseudo-period 14 glider stream. It uses two LWSS streams, a pentadecathlon and a volcano.

    .O...........................
    ..O..........OO..............
    OOO.........OOO..............
    ............OO.O.............
    .....O.......OOO.............
    ...O.O........O..............
    ....OO.......................
    .............................
    ......................OOOO...
    .....................OOOOOO..
    ....................OOOOOOOO.
    ............O......OO......OO
    ...OO.....O.O.......OOOOOOOO.
    .OO.OO.....OO........OOOOOO..
    .OOOO..........O......OOOO...
    ..OO............O............
    ..............OOO............
    .............................
    .............................
    .............................
    .....OOOOOOO.................
    ...OOO.OOO.OOO...............
    ..O....OOO....O..............
    ...OOOO.O.OOO.O..............
    .............O...............
    ..O.OO.O.O.O.................
    ..OO.O.O.O.OO................
    ......O..O.O.................
    .......OO..O.................
    ...........OO................

Glider injection gates are useful for building glider guns with pseudo-periods that are of the form nd, where n is a positive integer, and d is a proper divisor of some convenient base gun period (such as 30 or 46), with d > 13.

:glasses (p2) Compare scrubber and spark coil.

    ....O........O....
    ..OOO........OOO..
    .O..............O.
    .O..OOO....OOO..O.
    OO.O...O..O...O.OO
    ...O...OOOO...O...
    ...O...O..O...O...
    ....OOO....OOO....
    ..................
    ....OO.O..O.OO....
    ....O.OO..OO.O....

:glider (c/4 diagonally, p4) The smallest, most common and first discovered spaceship. This was found by Richard Guy in 1970 while Conway's group was attempting to track the evolution of the R-pentomino. The name is due in part to the fact that it is glide symmetric. (It is often stated that Conway discovered the glider, but he himself has said it was Guy. See also the cryptic reference ("some guy") in Winning Ways.)

    OOO
    O..
    .O.

The term "glider" is also occasionally (mis)used to mean "spaceship".

:glider-block cycle An infinite oscillator based on the following reaction (a variant of the rephaser). The oscillator consists of copies of this reaction displaced 2n spaces from one another (for some n>6) with blocks added between the copies in order to cause the reaction to occur again halfway through the period. The period of the resulting infinite oscillator is 8n-20. (Alternatively, in a cylindrical universe of width 2n the oscillator just consists of two gliders and two blocks.)

    ...OO...
    ...OO...
    ........
    ........
    ..O..O..
    O.O..O.O
    .OO..OO.

:glider constructible See glider synthesis.

:glider construction = glider synthesis.

:glider duplicator Any reaction in which one input glider is converted into two output gliders. This can be done by oscillators or spaceships, or by Herschel conduits or other signal circuitry such as the stable example shown under splitter. The most useful glider duplicators are those with low periods.

The following period 30 glider duplicator demonstrates a simple mechanism found by Dieter Leithner. The input glider stream comes in from the upper left, and the output glider streams leave at the upper and lower right. One of the output glider streams is inverted, so an inverting reflector is required to complete the duplicator. To produce non-parallel output, an inline inverter could be substituted for the northmost p30 glider gun.

    .......O....OO.......................OO.........
    ........O....O.......................OO.........
    ......OOO....O.O.......O..........OO......O...OO
    ..............OO.......O.O.......OOO.....O...O.O
    ..........................OO......OO......OOOOO.
    ..........................OO.........OO....OOO..
    ..........................OO.........OO.........
    .......................O.O......................
    .......................O........................
    ................................................
    ................................................
    ........................OO......................
    ........................OO......................
    ................................................
    ................................................
    ................................................
    .........................OOO....................
    ...........................O....................
    ..........................O.....................
    ................................................
    ................................................
    .........O.O....................................
    .......O...O.....OOO............................
    OO.....O.......O.O..O..OO.......................
    OO....O....O.......OO..O..O.....................
    .......O...................O....................
    .......O...O..OOO..........O....................
    .........O.O...............O....................
    .......................O..O.....OO..............
    .......................OO.......O.O.............
    ..................................O.............
    ..................................OO............

Spaceship convoys that can duplicate gliders are very useful since they (along with glider turners) provide a means to clean up many dirty puffers by duplicating and turning output gliders so as to impact into the exhaust to clean it up.

Glider duplicators and turners are known for backward gliders using p2 c/2 spaceships, and for forward gliders using p3 c/3 spaceships. These are the most general duplicators for these speeds.

:glider gun A gun that fires gliders. For examples, see Gosper glider gun, Simkin glider gun, new gun, p45 gun.

True-period glider guns are known for some low periods, and for all periods over 53 using Herschel conduit technology. See true for a list of known true-period guns. The lowest true-period gun possible is the p14 gun since that is the lowest possible period for any glider stream, but no example has yet been found.

Pseudo-period glider guns are known for every period above 13. These are made by using multiple true-period guns of some multiple of the period, and glider injection methods to fill in the gaps.

:glider injection gate = GIG

:glider lane See lane.

:gliderless A gun is said to be gliderless if it does not use gliders. The purist definition would insist that a glider does not appear anywhere, even incidentally. For a long time the only known way to construct LWSS, MWSS and HWSS guns involved gliders, and it was not until April 1996 that Dieter Leithner constructed the first gliderless gun (a p46 LWSS gun).

In October 2017 Matthias Merzenich used two copies of Tanner's p46 to create a p46 MWSS gun. This is the smallest known gliderless gun, and also the smallest known MWSS gun.

    ......O.................................
    ......OOO...............................
    .........O..............................
    ........OO..............................
    .....O..................................
    ...OOO..................................
    ..O.....................................
    ..OO....................................
    ........................................
    ..OO.......O.O..O.O.....................
    ..O.O......O..O...O.....................
    ...O.....OO...O.O.O.....................
    OOO.......OOO.O.........................
    O......OO.....OO........OOO.............
    .......OO....OO........O...O............
    .......OO...O.........O.....O...........
    .......OO.O.O........O...O...O..........
    .......OO.O.O........O.......O.....OO...
    .....................O.O...O.O.....OO...
    .................OO...OO...OO...........
    ..........OO....O.O.....................
    ......OO..OO....O...................OO..
    .....O.O.......OO...................O...
    .....O.............OO................OOO
    ....OO..............O....O.............O
    ....................O.O.O.O.............
    .....................OO.OO.O............
    ...........................O............
    ...........................OO...........

:glider pair Two gliders travelling in the same direction with a specific spacetime offset. In a transceiver the preferred term is tandem glider. For several years, glider pairs on lanes separated by 9 or 10 half diagonals were the standard building blocks in Geminoid construction arm recipes. In more recent 0hd and single-channel construction toolkits, all gliders share the same lane, but glider pairs and singletons are still important concepts.

:glider-producing switch engine See stabilized switch engine.

:glider pusher An arrangement of a queen bee shuttle and a pentadecathlon that can push the path of a passing glider out by one half-diagonal space. This was found by Dieter Leithner in December 1993 and is shown below. It is useful for constructing complex guns where it may be necessary to produce a number of gliders travelling on close parallel paths. See also edge shooter.

    .........OO..............
    .........OO..............
    .........................
    ..........O..............
    .........O.O.............
    .........O.O.............
    ..........O..............
    .........................
    .........................
    .......OO.O.OO...........
    .......O.....O...........
    ........O...O............
    .O.......OOO.............
    ..O......................
    OOO......................
    .........................
    .........................
    .................O....O..
    ...............OO.OOOO.OO
    .................O....O..

:glider recipe = glider synthesis.

:glider reflector See reflector.

:gliders by the dozen (stabilizes at time 184) In early references this is usually shown in a larger form whose generation 1 is generation 8 of the form shown here.

    OO..O
    O...O
    O..OO

:glider stopper A Spartan logic circuit discovered by Paul Callahan in 1996. It allows a glider signal to pass through the circuit, leaving behind a beehive that can cleanly absorb a single glider from a perpendicular glider stream. Two optional glider outputs are also shown. The circuit can't be re-used until the beehive "bit" is cleared by the passage of at least one perpendicular input. A similar mechanism discovered more recently is shown in the beehive stopper entry.

    .O...........................................
    ..O..........................................
    OOO..........................................
    .............................................
    .............................................
    ...................................O.........
    ..................................O..........
    ..................................OOO........
    .............................................
    ...............................O.............
    ...............................O.O...........
    ...................OO..........OO............
    ...................OO........................
    .............................................
    .............................................
    .............................................
    .............................................
    .............................................
    .............................................
    .............................................
    ...................OO........................
    ..................O..O.......................
    ...................OO........................
    ..........................OO.................
    ..........................OO.................
    ...........................................OO
    ........OO.................................O.
    .......O.O...............................O.O.
    .......O.................................OO..
    ......OO.....................................
    .............................................
    .............................................
    .............................................
    .................OO..........................
    ................O.O..........................
    ................O............................
    ...............OO............................

:glider synthesis Construction of an object by means of glider collisions. It is generally assumed that the gliders should be arranged so that they could come from infinity. That is, gliders should not have had to pass through one another to achieve the initial arrangement.

Glider syntheses for all still lifes and known oscillators with at most 14 cells were found by Dave Buckingham. As of June 2018, this limit has been increased to 18 cells.

Perhaps the most interesting glider syntheses are those of spaceships, because these can be used to create corresponding guns and rakes. Many of the c/2 spaceships that are based on standard spaceships have been synthesized, mostly by Mark Niemiec. In June 1998 Stephen Silver found syntheses for some of the Corderships (although it was not until July 1999 that Jason Summers used this to build a Cordership gun). In May 2000, Noam Elkies suggested that a 2c/5 spaceship found by Tim Coe in May 1996 might be a candidate for glider synthesis. Initial attempts to construct a synthesis for this spaceship got fairly close, but it was only in March 2003 that Summers and Elkies managed to find a way to perform the crucial last step. Summers then used the new synthesis to build a c/2 forward rake for the 2c/5 spaceship; this was the first example in Life of a rake which fires spaceships that travel in the same direction as the rake but more slowly.

A 3-glider synthesis of a pentadecathlon is shown in the diagram below. This was found in April 1997 by Heinrich Koenig and came as a surprise, as it was widely assumed that anything using just three gliders would already be known.

    ......O...
    ......O.O.
    ......OO..
    ..........
    OOO.......
    ..O.......
    .O.....OO.
    ........OO
    .......O..

:glider to 2 blocks A converter discovered by Sergei Petrov on 8 October 2011, used in his later G4 receiver.

    ..........O.......OO
    OO......O.O.......OO
    OO.......OO.........
    ....................
    ....................
    ....................
    ...........OO.......
    ...........OO.......
    ....................
    ....................
    ..............OO....
    ...............O....
    ...............O.O..
    ................OO..
    ....................
    ....................
    ....................
    ........OO..........
    ........OO..........

:glider to block A converter discovered by Sergei Petrov that places a block at its right edge in response to a single glider input. This has a variety of uses in Herschel circuitry and other signal-processing applications.

    ...........O....
    .....O.....OOO..
    .....OOO......O.
    ........O....OO.
    .......OO.......
    ................
    ................
    ................
    ................
    ................
    OO..............
    OO..........OO..
    ............O.O.
    .............O..
    ................
    ................
    ................
    ............OO..
    ..OOO.......O...
    ....O........OOO
    ...O...........O

:glider train A certain p64 c/2 orthogonal puffer that produces two rows of blocks and two backward glider waves. Ten of these were used to make the first breeder.

    ..............................O............
    ...............................O...........
    .........................O.....O...........
    ....O.....................OOOOOO.....OOOOOO
    .....O..............................O.....O
    O....O....................................O
    .OOOOO..............................O....O.
    ......................................OO...
    ...........................................
    .....................................O.....
    ....................................O......
    ...................................OO...OO.
    ...................................O.O...OO
    ....................................O...OO.
    ........................................O..
    ...........................................
    ........................................O..
    ....................................O...OO.
    ...................................O.O...OO
    ...................................OO...OO.
    ....................................O......
    .....................................O.....
    ...........................................
    ......................................OO...
    .OOOOO..............................O....O.
    O....O....................................O
    .....O..............................O.....O
    ....O.....................OOOOOO.....OOOOOO
    .........................O.....O...........
    ...............................O...........
    ..............................O............

:glider turner Any reaction in which a glider is turned onto a new path by a spaceship, oscillator, or still life constellation. In the last two cases, the glider turner is usually called a reflector if the reaction is repeatable, or a one-time turner if the reaction can only happen once.

Glider turners are easily built using standard spaceships. The following diagram shows a convoy which turns a forward glider 90 degrees, with the new glider also moving forwards.

    .........OO.........
    ........OO.OOOO.....
    .O.......OOOOOO.....
    O.........OOOO......
    OOO.................
    ....................
    ....................
    ....................
    ....................
    ...O................
    .O...O..............
    O...................
    O....O..............
    OOOOO...............
    ....................
    ....................
    .............OOOOOO.
    .............O.....O
    .............O......
    ..............O....O
    ................OO..

Small rearrangements of the back two spaceships can alternatively send the output glider into any of the other three directions.

See also glider duplicator and reflector.

:glide symmetric Undergoing simultaneous reflection and translation. A glide symmetric spaceship is sometimes called a flipper.

:Gn An abbreviation specific to converters that produce multiple gliders. A "G" followed by any integer value means that the converter produces a tandem glider - two parallel glider outputs with lanes separated by the specified number of half diagonals.

:gnome = fox

:GoE = Garden of Eden

:GoL = Game of Life

:Golly A cross-platform open source Life program by Andrew Trevorrow and Tomas Rokicki. Unlike most Life programs it includes the ability to run patterns using the hashlife algorithm. It is available from http://golly.sourceforge.net.

:Gosper glider gun The first known gun, and indeed the first known finite pattern displaying infinite growth, found by Bill Gosper in November 1970. This period 30 gun remains the smallest known gun in terms of its bounding box, though some variants of the p120 Simkin glider gun have a lower population. Gosper later constructed several other guns, such as new gun and the p144 gun shown under factory. See also p30 gun.

    ........................O...........
    ......................O.O...........
    ............OO......OO............OO
    ...........O...O....OO............OO
    OO........O.....O...OO..............
    OO........O...O.OO....O.O...........
    ..........O.....O.......O...........
    ...........O...O....................
    ............OO......................

:Gotts dots A 41-cell 187x39 superlinear growth pattern found by Bill Gosper in March 2006, who named it in honour of Nick Gotts, discoverer of many other low-population superlinear patterns, such as Jaws, the mosquitoes, teeth, catacryst and metacatacryst. See switch-engine ping-pong for the lowest-population superlinear growth pattern as of July 2018, along with a list of the record-holders.

Collisions within the pattern cause it to sprout its Nth switch engine at generation T = ~224n-6. The population of the pattern at time t is asymptotically proportional to t times log(t), so the growth rate is O(t ln(t)), faster than linear growth but slower than quadratic growth.

:gourmet (p32) Found by Dave Buckingham in March 1978. Compare with pi portraitor and popover.

    ..........OO........
    ..........O.........
    ....OO.OO.O....OO...
    ..O..O.O.O.....O....
    ..OO....O........O..
    ................OO..
    ....................
    ................OO..
    O.........OOO..O.O..
    OOO.......O.O...O...
    ...O......O.O....OOO
    ..O.O..............O
    ..OO................
    ....................
    ..OO................
    ..O........O....OO..
    ....O.....O.O.O..O..
    ...OO....O.OO.OO....
    .........O..........
    ........OO..........

:gp = glider pair

:GPSE = glider-producing switch engine

:grammar A set of rules for connecting components together to make an object such as a spaceship, oscillator or still life. For example, in August 1989 Dean Hickerson found a grammar for constructing an infinite number of short wide c/3 period 3 spaceships, using 33 different components and a table showing the ways that they can be joined together.

:grandfather = grandparent

:grandfatherless A traditional name for a pattern with one or more parents but no grandparent. This was a hypothetical designation until May 2016. See grandparent for details.

:grandparent A pattern is said to be a grandparent of the pattern it gives rise to after two generations. For over thirty years, a well-known open problem was the question of whether any pattern existed that had a parent but no grandparent. In 1972, LifeLine Volume 6 mentioned John Conway's offer of a $50 prize for a solution to the problem, but it remained open until May 2016 when a user with the conwaylife.com forum handle 'mtve' posted an example.

Other patterns have since been found that have a grandparent but no great-grandparent, or a great-grandparent but no great-great-grandparent. Further examples in this series almost certainly exist, but as of July 2018 none have yet been found.

:Gray counter (p4) Found in 1971. If you look at this in the right way you will see that it cycles through the Gray codes from 0 to 3. Compare with R2D2.

    ......O......
    .....O.O.....
    ....O.O.O....
    .O..O...O..O.
    O.O.O...O.O.O
    .O..O...O..O.
    ....O.O.O....
    .....O.O.....
    ......O......

:gray ship = grey ship

:great on-off (p2)

    ..OO....
    .O..O...
    .O.O....
    OO.O..O.
    ....OO.O
    .......O
    ....OOO.
    ....O...

:grey counter = Gray counter (This form is erroneous, as Gray is surname, not a color.)

:grey ship A spaceship that contains a region with an average density of 1/2, and which is extensible in such a way that the region of average density 1/2 can be made larger than any given square region.

See also with-the-grain grey ship, against-the-grain grey ship and hybrid grey ship.

:grin The following common parent of the block. This name relates to the infamous Cheshire cat. See also pre-block.

    O..O
    .OO.

:grow-by-one object A pattern whose population increases by one cell every generation. The smallest known grow-by-one object is the following 44-cell pattern (David Bell's one-cell improvement of a pattern found by Nicolay Beluchenko, September 2005).

    ........OO.......
    .......OO........
    .........O.......
    ...........OO....
    ..........O......
    .................
    .........O..OO...
    .OO.....OO....O..
    OO.....O.....O...
    ..O....O.O...OO..
    ....O..O....OO.O.
    ....OO.......OO..
    ........O....O.OO
    .......O.O..O.OO.
    ........O........

:growing/shrinking line ship A line ship in which the line repeatedly grows and shrinks, resulting in a high-period spaceship.

:growing spaceship An object that moves like a spaceship, except that its front part moves faster than its back part and a wick extends between the two. Put another way, a growing spaceship is a puffer whose output is burning cleanly at a slower rate than the puffer is producing it. Examples include blinker ships, pi ships, and some wavestretchers.

:G-to-H A converter that takes a glider as an input signal and produces a Herschel output, which can then be used by other conduits. G-to-Hs are frequently used in stable logic circuitry. Early examples include Callahan G-to-H, Silver G-to-H, and p8 G-to-H for periodic circuits. A more compact recent example is the syringe.

:gull = elevener

:gun Any stationary pattern that emits spaceships (or rakes) forever. For examples see double-barrelled, edge shooter, factory, gliderless, Gosper glider gun, Simkin glider gun, new gun and true.

:gunstar Any of a series of glider guns of period 144+72n (for all non-negative integers n) constructed by Dave Buckingham in 1990 based on his transparent block reaction and Robert Wainwright's p72 oscillator (shown under factory).

:gutter A single straight line of cells along the axis of symmetry of a mirror-symmetric pattern. Most commonly this is an orthogonal line, and the pattern is then odd-symmetric (as opposed to even-symmetric, where the axis of symmetry follows the boundary between two rows or columns of cells).

The birth rule for Conway's Life trivially implies that if there are no live cells in the gutter of a symmetric pattern, new cells can never be born there. For examples, see 44P5H2V0, 60P5H2V0, Achim's p4, brain, c/6 spaceship, centinal, p54 shuttle, pufferfish, snail, spider, and pulsar (in two orientations).

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