(A more realistic kneading process would fold material rather than cutting it, but the same sensitive dependence on initial conditions would occur.)

The triangles in the construction have angles which are multiples of π /5 , so that the whole tiling has an approximate 5-fold symmetry (see page 994 ). Repetitive tilings of the plane can only have 3-, 4- or 6-fold symmetry.

[NP completeness in] natural systems Finding minimum energy configurations is formally NP-complete in standard models of natural systems such as folding protein and DNA molecules (see page 1003 ), collections of charges on a sphere (compare page 987 ), and finite regions of spin glasses (see page 944 ).

With this setup successive steps in the evolution of the system can be obtained from GMAStep[rules_, {list_, nlist_}] := Module[{a, na}, {a, na} = Transpose[Map[Replace[Take[list, {# - 1, # + 1}], rules]&, nlist]]; {Fold[ReplacePart[#, Last[#2], First[#2]]&, list, Transpose[{nlist, a}]], Union[Flatten[nlist + na]]}]

Runs of digits [in numbers] One can consider any base 2 digit sequence as consisting of successive runs of 0's and 1's, constructed from the list of run lengths by Fold[Join[#1, Table[1 - Last[#1], {#2}]] &, {0}, list] This representation is related to so-called surreal numbers (though with the first few digits different).

Random walks In one dimension, a random walk with t steps of length 1 starting at position 0 can be generated from NestList[(# + (-1)^Random[Integer])&, 0, t] or equivalently FoldList[Plus, 0, Table[(-1)^Random[Integer], {t}]] A generalization to d dimensions is then FoldList[Plus, Table[0, {d}], Table[RotateLeft[PadLeft[ {(-1)^Random[Integer]}, d], Random[Integer, d - 1]], {t}]] A fundamental property of random walks is that after t steps the root mean square displacement from the starting position is proportional to √ t . … To make a random walk on a lattice with k directions in two dimensions, one can set up e = Table[{Cos[2 π s/k], Sin[2 π s/k]}, {s, 0, k - 1}] then use FoldList[Plus, {0, 0}, Table[e 〚 Random[Integer, {1, k}] 〛 , {t}]] It turns out that on any regular lattice, in any number of dimensions, the average behavior of a random walk is always isotropic.

The evolution of the system for t steps can be obtained from SSEvolve[rule_, init_, t_, d_Integer] := Nest[FlattenArray[# /. rule, d] &, init, t] FlattenArray[list_, d_] := Fold[Function[{a, n}, Map[MapThread[Join, #, n] &, a, -{d + 2}]], list, Reverse[Range[d] - 1]] The analog in 3D of the 2D rule on page 187 is {1  Array[If[LessEqual[##], 0, 1] &, {2, 2, 2}], 0  Array[0 &, {2, 2, 2}]} Note that in d dimensions, each black cell must be replaced by at least d + 1 black cells at each step in order to obtain an object that is not restricted to a dimension d - 1 hyperplane.

In case (c), the following gives a list of the numbers of nodes generated up to step t : FoldList[Plus, 1, Join[{1, 4, 12, 10, -20, 6, 4}, Map[d, IntegerDigits[Range[4, t - 5], 2]]]] d[{___, 1}] = 1 d[{1, p : 0 .., 0}] := -Apply[Plus, 4 Range[Length[{p}]] - 1] + 6 d[{__, 1, p : 0 .., 0}] := d[{1, p, 0}] - 7 d[{___, p : 1 .., q : 0 ..., 1, 0}] := 4 Length[{p}] + 3 Length[{q}] + 2 d[{___, p : 1 .., 1, 0}] := 4 Length[{p}] + 2

Applying FoldList[Plus, 0, 2list - 1] to the whole sequence yields the pattern shown below. … This is similar to picture (c) on page 131 , and is a digit-by-digit version of FoldList[Plus, 0, Table[Apply[Plus, 2 Rest[IntegerDigits[i, 2]] - 1], {i, n}]] Note that although the picture above has a nested structure, the original concatenation sequences are not nested, and so cannot be generated by substitution systems.

Substitution systems in which all replacements are done that are found to fit in a left-to-right scan can be implemented as follows GSSEvolveList[rule_, s_, n_] := NestList[GSSStep[rule, #] &, s, n] GSSStep[rule_, s_] := g[rule, s, f[StringPosition[s, Map[First, rule]]]] f[{ }] = { }; f[s_] := Fold[If[Last[Last[#1]] ≥ First[#2], #1, Append[#1, #2]]&, {First[s]}, Rest[s]] g[rule_, s_, { }] := s; g[rule_, s_, pos_] := StringReplacePart[ s, Map[StringTake[s, #] &, pos] /. rule, pos] with rules given as {"ABA"  "BAAB", "BBBB"  "AA"} .