**Basics**

A neighborhood is a set of ordered integer pairs (x,y), each corresponding to a cell on a grid. (x = right, y = up) (0,0) is defined as the center; a neighborhood containing the center is called closed, otherwise it is open. The empty neighborhood is denoted 0. The neighborhood containing only the center is denoted 1. A neighborhood containing only one cell is called singular.

**Operators**

Basic operators: Set union, subtraction, intersection, and XOR, expressed as X + Y, X - Y, X & Y, X @ Y.

Code: Select all

`x = 27, y = 25, rule = LifeHistory`

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3F3.2A$6.2F!

Multiplication: For all pairs of cells (a,b) and (c,d) in neighborhoods X and Y respectively, the product of X and Y (expressed X * Y) contains cell (a+c,b+d).

Code: Select all

`x = 56, y = 25, rule = LifeHistory`

53.2A$5.F.F3.A3.3F6.A6.2A2.F.F3.A3.3F4.2A$.DA3.F4.D11.D6.AE4.F4.D11.E

$5.F.F7.3F13.A3.F.F7.3F7$53.2A$5.F.F3.A3.3F5.2A6.2A2.F.F3.A3.3F4.3A$.

EA3.F4.E11.EA5.AE4.F4.E10.AE$5.F.F7.3F13.A3.F.F7.3F5.A7$22.3A27.4A$.A

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8.2AE2A$.A3.F.F2.3A2.3F3.5A5.A3.F.F2.3A2.3F3.4A$22.3A27.3A!

Code: Select all

`x = 55, y = 26, rule = LifeHistory`

4.F.F$4.F.F24.A8.3A7.A$5.F5.DA8.EA7.AEA7.AEA8.EA$4.F.F24.A8.3A7.2A$4.

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3A4.2AE3A$F.F26.5A4.7A3.6A$F.F27.3A5.7A4.4A$31.A6.7A3.4A!

Code: Select all

```
x = 56, y = 26, rule = LifeHistory
5.F.F28.F.F$.2A2.3F3.A3.3F4.A9.2A2.3F7.3F5.2A$AE3.F.F3.D10.AEA6.AE3.F
.F2.ADA9.AEA$.A3.3F7.3F5.A8.A3.3F7.3F4.2A$5.F.F28.F.F6$5.F.F15.A12.F.
F15.A$.2A2.3F4.A2.3F4.A9.2A2.3F4.A2.3F4.3A$AE3.F.F3.D11.E7.AE3.F.F3.D
A9.AE$.A3.3F7.3F4.A.A7.A3.3F7.3F4.3A$5.F.F28.F.F5$55.A$5.F.F15.A.A10.
F.F14.A$.2A2.3F7.3F14.2A2.3F5.A.3F$AE3.F.F3.D.A7.A.E7.AE3.F.F3.D11.E$
.A3.3F7.3F14.A3.3F7.3F3.A$5.F.F15.A12.F.F$55.A!
```

**Identities**

For any neighborhoods X and Y and any singular neighborhood S = {(x,y)}:

X + Y = Y + X

X & Y = Y & X

X @ Y = Y @ X

X + 0 = X - 0 = X @ 0 = X

X & 0 = 0

0 - X = 0

X + X = X & X = X

X - X = X @ X = 0

|X + Y| <= |X| + |Y|

|0| = 0

|1| = |S| = 1

X * Y = Y * X

X * 0 = 0

X * 1 = X

X * S = X translated right x and up y

X * Y is closed if both X and Y are closed

|X * Y| <= |X| * |Y|

|X * S| = |X|

0 ^ n = 0

1 ^ n = 1

{(x,y)} ^ n = {(nx,ny)}

X ^ n is closed if X is closed

|X ^ n| <= |X| * n^2 (unproven)

X # Y = Y # X

X # 0 = 0

X # 1 = 1

X # r = X

X # Y is closed iff either X or Y are closed

|X # Y| <= |X| * |Y|

|X # S| = |X|

**Notable neighborhoods**

The elementary singular neighborhoods: r = {(1,0)}, u = {(0,1)}, l = {(-1,0)}, d = {(0,-1)} and their closed forms: R = {(0,0),(1,0)}, U = {(0,0),(0,1)}, L = {(0,0),(-1,0)}, D = {(0,0),(0,-1)}. Can be multiplied and added to form all neighborhoods.

Symmetric neighborhoods: h = l + r, H = L + R, v = u + d, V = U + D, n = u + d + l + r, N = n + 1, m = ul + u + ur + l + r + dl + d + dr, M = m + 1

Code: Select all

`x = 43, y = 19, rule = LifeHistory`

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2.F.F2.3F$.F3.F.F2.F.F2.F4.2F3.3F2.F.F2.F.F2.F.F$.F3.F.F2.F.F2.F4.F.F

2.F.F2.F.F2.F.F2.F.F$3F3.2F2.2F3.3F2.F.F2.F.F3.F3.F.F2.F.F2$6.A24.A4.

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4.F4.F.F2.F.F2.F.F2.3F$2F4.2F3.2F3.2F2.F4.F.F3.F3.F.F2.F.F2$6.A24.A4.

A3.3A$.D4.D4.D3.AD4.DA2.ADA3.D3.ADA2.ADA$11.A19.A4.A3.3A!