# Isotropic non-totalistic Life-like cellular automaton

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An isotropic non-totalistic Life-like cellular automaton is a generalization of the concept of a Life-like cellular automaton in which transitions take into account not only the total number of live neighbors of a cell, but also the relative configuration of those neighbors.

Isotropic non-totalistic rules are described using Hensel notation, an extension of B/S notation developed by Alan Hensel additionally describing allowed or forbidden configurations. Each digit in the rule's birth and survival conditions is followed by an optional suffix, with each allowed configuration described by a specific letter; a minus sign may be used to forbid configurations rather than allow them. If no configurations are specified, all are considered to be allowed, as in the totalistic case. This notation is not used by non-isotropic Life-like cellular automata.

For instance, B2-a/S12 (the "Just Friends" rule) indicates that a live cell will survive on 1 or 2 neighbors, or a dead cell get born on 2 neighbors, except when they are adjacent.

This notation has the following symmetry: For any letter x and number n≠4, nx is defined if and only (8-n)x is defined and moreover (8-n)x is the complement (change live cells to dead and dead cells to live; ignore the center cell) of nx.

The following table describes all possible neighborhood configurations for the Moore neighbourhood; where appropriate, the same configurations apply to the von Neumann neighbourhood:

Rules using the von Neumann neighbourhood can be simulated via isotropic non-totalistic rules on the Moore neighbourhood; for example, B1/SV becomes B1e2ak3inqy4ny5e/S.

## Hexagonal neighbourhood

It is possible to define isotropic non-totalistic Life-like CAs on a hexagonal grid as well. The following table describes all possible neighborhood configurations for the hexagonal neighbourhood, using notation due to Paul Callahan;[1][2] the names ortho, meta and para were chosen in analogy to arene substitution patterns in aromatic chemistry:

Golly does not support isotropic non-totalistic hexagonal rules using this syntax, so they must instead be simulated using either rule tables or MAP strings.

## Soup-searching non-totalistic rules

Adam P. Goucher's apgsearch was modified to support isotropic non-totalistic rules by Aidan F. Pierce on December 17, 2015.[3] Catagolue gained the ability to census isotropic non-totalistic rules in late January 2016.[4] apgsearch (apgluxe) 4.2, released on September 10, 2017, can search isotropic non-totalistic rules without B0.

## References

1. Paul Callahan (December 3, 1997). "Experiments with a somewhat "Life-like" hexagonal CA (long)". Retrieved on September 29, 2017.
2. http://www.conwaylife.com/forums/download/file.php?id=261
3. Aidan F. Pierce (December 17, 2015). "Re: Hacking apgsearch". ConwayLife.com forums. Retrieved on June 12, 2016.
4. Adam P. Goucher (January 21, 2016). "Re: apgsearch v2.2". ConwayLife.com forums. Retrieved on June 12, 2016.