Provided by: libmath-planepath-perl_122-1_all 
NAME
Math::PlanePath::SierpinskiTriangle -- self-similar triangular path traversal
SYNOPSIS
use Math::PlanePath::SierpinskiTriangle;
my $path = Math::PlanePath::SierpinskiTriangle->new;
my ($x, $y) = $path->n_to_xy (123);
DESCRIPTION
This path is an integer version of Sierpinski's triangle from
Waclaw Sierpinski, "Sur une Courbe Dont Tout Point est un Point de Ramification",
Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences, volume 160,
January-June 1915, pages 302-305.
<http://gallica.bnf.fr/ark:/12148/bpt6k31131/f302.image.langEN>
Unit triangles are numbered numbered horizontally across each row.
65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 15
57 58 59 60 61 62 63 64 14
49 50 51 52 53 54 55 56 13
45 46 47 48 12
37 38 39 40 41 42 43 44 11
33 34 35 36 10
29 30 31 32 9
27 28 8
19 20 21 22 23 24 25 26 7
15 16 17 18 6
11 12 13 14 5
9 10 4
5 6 7 8 3
3 4 2
1 2 1
0 <- Y=0
X= ... -9-8-7-6-5-4-3-2-1 0 1 2 3 4 5 6 7 8 9 ...
The base figure is the first two rows shape N=0 to N=2. Notice the middle "." position
X=0,Y=1 is skipped
1 . 2
0
This is replicated twice in the next row pair as N=3 to N=8. Then the resulting four-row
shape is replicated twice again in the next four-row group as N=9 to N=26, etc.
See the "SierpinskiArrowheadCentres" path to traverse by a connected sequence rather than
rows jumping across gaps.
Row Ranges
The number of points in each row is always a power of 2. The power is the count of 1-bits
in Y. (This count is sometimes called Gould's sequence.)
rowpoints(Y) = 2^count_1_bits(Y)
For example Y=13 is binary 1101 which has three 1-bits so in row Y=13 there are 2^3=8
points.
Because the first point is N=0, the N at the left of each row is the cumulative count of
preceding points,
Ndepth(Y) = rowpoints(0) + ... + rowpoints(Y-1)
Since the powers of 2 are always even except for 2^0=1 in row Y=0, this Ndepth(Y) total is
always odd. The self-similar nature of the triangle means the same is true of the sub-
triangles, for example odd N=31,35,41,47,etc on the left of the triangle at X=8,Y=8. This
means in particular the primes (being odd) fall predominately on the left side of the
triangles and sub-triangles.
Replication Sizes
Counting the single point N=0 as level=0, then N=0,1,2 as level 1, each replication level
goes from
Nstart = 0
Nlevel = 3^level - 1 inclusive
For example level 2 is from N=0 to N=3^2-1=8. Each level doubles in size,
0 <= Y <= 2^level - 1
- 2^level + 1 <= X <= 2^level - 1
Align Right
Optional "align=>"right"" puts points to the right of the Y axis, packed next to each
other and so using an eighth of the plane.
align => "right"
7 | 19 20 21 22 23 24 25 26
6 | 15 16 17 18
5 | 11 12 13 14
4 | 9 10
3 | 5 6 7 8
2 | 3 4
1 | 1 2
Y=0 | 0
+-------------------------
X=0 1 2 3 4 5 6 7
Align Left
Optional "align=>"left"" puts points to the left of the Y axis, ie. into negative X. The
rows are still numbered starting from the left, so it's a shift across, not a negate of X.
align => "left"
19 20 21 22 23 24 25 26 | 7
15 16 17 18 | 6
11 12 13 14 | 5
9 10 | 4
5 6 7 8 | 3
3 4 | 2
1 2 | 1
0 | <- Y=0
-------------------------+
-7 -6 -5 -4 -3 -2 -1 X=0
Align Diagonal
Optional "align=>"diagonal"" puts rows on diagonals down from the Y axis to the X axis.
This uses the whole of the first quadrant, with gaps according to the pattern.
align => "diagonal"
15 | 65 ...
14 | 57 66
13 | 49 67
12 | 45 50 58 68
11 | 37 69
10 | 33 38 59 70
9 | 29 39 51 71
8 | 27 30 34 40 46 52 60 72
7 | 19 73
6 | 15 20 61 74
5 | 11 21 53 75
4 | 9 12 16 22 47 54 62 76
3 | 5 23 41 77 ...
2 | 3 6 17 24 35 42 63 78
1 | 1 7 13 25 31 43 55 79
Y=0 | 0 2 4 8 10 14 18 26 28 32 36 44 48 56 64 80
+-------------------------------------------------
X=0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
This form visits all points X,Y where X and Y written in binary have no 1-bits in the same
bit positions, ie. where X bitand Y == 0. For example X=13,Y=3 is not visited because
13="1011" and 6="0110" both have bit "0010" set.
This bit-and rule is an easy way to test for visited or not visited cells of the pattern.
The visited cells can be calculated by this diagonal X,Y bitand, but then plotted X,X+Y
for the "right" align or X-Y,X+Y for "triangular".
Cellular Automaton
The triangle arises in Stephen Wolfram's 1-D cellular automatons (per
Math::PlanePath::CellularRule and Cellular::Automata::Wolfram).
align rule
----- ----
"triangular" 18,26,82,90,146,154,210,218
"right" 60
"left" 102
<http://mathworld.wolfram.com/Rule90.html>
<http://mathworld.wolfram.com/Rule60.html>
<http://mathworld.wolfram.com/Rule102.html>
In each row the rule 18 etc pattern turns a cell "on" in the next row if one but not both
its diagonal predecessors are "on". This is a mod 2 sum giving Pascal's triangle mod 2.
Some other cellular rules are variations on the triangle,
• Rule 22 is "triangular" but filling the gap between leaf points such as N=5 and N=6.
• Rule 126 adds an extra point on the inward side of each visited.
• Rule 182 fills in the big gaps leaving just a single-cell empty border delimiting
them.
N Start
The default is to number points starting N=0 as shown above. An optional "n_start"
parameter can give a different start, with the same shape. For example starting at 1,
which is the numbering of "CellularRule" rule=60,
n_start => 1
20 21 22 23 24 25 26 27
16 17 18 19
12 13 14 15
10 11
6 7 8 9
4 5
2 3
1
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
"$path = Math::PlanePath::SierpinskiTriangle->new ()"
"$path = Math::PlanePath::SierpinskiTriangle->new (align => $str, n_start => $n)"
Create and return a new path object. "align" is a string, one of the following as
described above.
"triangular" (the default)
"right"
"left"
"diagonal"
Descriptive Methods
"$n = $path->n_start()"
Return the first N in the path. This is 0 by default, or the given "n_start"
parameter.
Tree Methods
"@n_children = $path->tree_n_children($n)"
Return the children of $n, or an empty list if "$n < n_start" (ie. before the start of
the path).
The children are the points diagonally up left and right on the next row (Y+1). There
can be 0, 1 or 2 such points. At even depth there's 2, on depth=1mod4 there's 1. On
depth=3mod4 there's some 0s and some 1s. See "N to Number of Children" below.
For example N=3 has two children N=5,N=6. Then in turn N=5 has just one child N=9 and
N=6 has no children. The way points are numbered across a row means that when there's
two children they're consecutive N values.
"$n_parent = $path->tree_n_parent($n)"
Return the parent node of $n, or "undef" if "$n <= n_start" (the top of the triangle).
"$depth = $path->tree_n_to_depth($n)"
Return the depth of node $n, or "undef" if there's no point $n. In the "triangular",
"right" and "left" alignments this is the same as the Y coordinate from "n_to_xy()".
In the "diagonal" alignment it's X+Y.
"$n = $path->tree_depth_to_n($depth)"
"$n = $path->tree_depth_to_n_end($depth)"
Return the first or last N at tree level $depth. The start of the tree is depth=0 at
the origin X=0,Y=0.
This is the N at the left end of each row. So in the default triangular alignment
it's the same as "xy_to_n(-$depth,$depth)".
Level Methods
"($n_lo, $n_hi) = $path->level_to_n_range($level)"
Return "(0, 3**$level - 1)".
FORMULAS
X,Y to N
For calculation it's convenient to turn the X,Y coordinates into the "right" alignment
style, so that Y is the depth and X is in the range 0<=X<=Y.
The starting position of each row of the triangle is given by turning 1-bits of the depth
into powers-of-3.
Y = depth = 2^a + 2^b + 2^c + 2^d ... a>b>c>d...
Ndepth = first N at this depth
= 3^a
+ 2 * 3^b
+ 2^2 * 3^c
+ 2^3 * 3^d
+ ...
For example depth=6=2^2+2^1 starts at Ndepth=3^2+2*3^1=15. The powers-of-3 are the three
parts of the triangle replication. The power-of-2 doubling is the doubling of the row Y
when replicated.
Then the bits of X at the positions of the 1-bits of the depth become an N offset into the
row.
a b c d
depth = 10010010010 binary
X = m00n00p00q0
Noffset = mnpq binary
N = Ndepth + Noffset
For example in depth=6 binary "110" then at X=4="100" take the bits of X where depth has
1-bits, which is X="10_" so Noffset="10" binary and N=15+2=17, as per the "right" table
above at X=4,Y=6.
If X has any 1-bits which are a 0-bits in the depth depth then that X,Y is not visited.
For example if depth=6="110" then X=3="11" is not visited because the low bit X="__1" has
depth="__0" at that position.
N to Depth
The row containing N can be found by working down the Ndepth formula shown above. The "a"
term is the highest 3^a <= N, thus giving a bit 2^a for the depth. Then for the remaining
Nrem = N - 3^a find the highest "b" where 2*3^b <= Nrem. And so on until reaching an Nrem
which is too small to subtract any more terms.
It's convenient to go by bits high to low of the prospective depth, deciding at each bit
whether Nrem is big enough to give the depth a 1-bit there, or whether it must be a 0-bit.
a = floor(log3(N)) round down to power-of-3
pow = 3^a
Nrem = N - pow
depth = high 1-bit at bit position "a" (counting from 0)
factor = 2
loop bitpos a-1 down to 0
pow /= 3
if pow*factor <= Nrem
then depth 0-bit, factor *= 2
else depth 1-bit
factor is 2^count1bits(depth)
Noffset = Nrem offset into row
0 <= Noffset < factor
N to X,Y
N is turned into depth and Noffset as per above. X in "right" alignment style is formed
by spreading the bits of Noffset out according to the 1-bits of the depth.
depth = 100110 binary
Noffset = abc binary
Xright = a00bc0
For example in depth=5 this spreads an Noffset=0to3 to make X=000, 001, 100, 101 in binary
and in "right" alignment style.
From an X,Y in "right" alignment the other alignments are formed
alignment from "right" X,Y
--------- ----------------
triangular 2*X-Y, Y so -Y <= X < Y
right X, Y unchanged
left X-Y, Y so -Y <= X <= 0
diagonal X, Y-X downwards sloping
N to Number of Children
The number of children follows a pattern based on the depth.
depth number of children
----- ------------------
12 2 2 2 2
11 1 0 0 1 1 0 0 1
10 2 2 2 2
9 1 1 1 1
8 2 2
7 1 0 0 0 0 0 0 1
6 2 2 2 2
5 1 1 1 1
4 2 2
3 1 0 0 1
2 2 2
1 1 1
0 2
If depth is even then all points have 2 children. For example row depth=6 has 4 points
and all have 2 children each.
At odd depth the number of children is either 1 or 0 according to the Noffset position in
the row masked down by the trailing 1-bits of the depth.
depth = ...011111 in binary, its trailing 1s
Noffset = ...00000 \ num children = 1
= ...11111 /
= ...other num children = 0
For example depth=11 is binary "1011" which has low 1-bits "11". If those two low bits of
Noffset are "00" or "11" then 1 child. Any other bit pattern in Noffset ("01" or "10" in
this case) is 0 children. Hence the pattern 1,0,0,1,1,0,0,1 reading across the depth=11
row.
In general when the depth doubles the triangle is replicated twice and the number of
children is carried with the replications, except the middle two points are 0 children.
For example the triangle of depth=0to3 is repeated twice to make depth=4to7, but the
depth=7 row is not children 10011001 of a plain doubling from the depth=3 row, but instead
10000001 which is the middle two points becoming 0.
N to Number of Siblings
The number of siblings of a given node is determined by its depth,
depth number of siblings
----- ------------------
4 0 0
3 1 1 1 1
2 0 0
1 1 1
0 0
depth number of siblings
----- ------------------
odd 1
even 0
In an even row the points are all spread apart so there are no siblings. The points in
such a row are cousins or second cousins, etc, but none share a parent.
In an odd row each parent node (an even row) has 2 children and so each of those points
has 1 sibling.
The effect is to conflate the NumChildren=1 and NumChildren=0 cases in the picture above,
those two becoming a single sibling.
num children of N num siblings of N
----------------- -----------------
0 or 1 1
2 0
Rectangle to N Range
An easy range can be had just from the Y range by noting the diagonals X=Y and X=-Y are
always visited, so just take the Ndepth of Ymin and Nend of Ymax,
# align="triangular"
Nmin = N at X=-Ymin,Y=Ymin
Nmax = N at X=Ymax,Y=Ymax
Or in "right" style the left end is at X=0 instead,
# align="right"
Nmin = N at X=0,Ymin
Nmax = N at Ymax,Ymax
For less work but a bigger over-estimate, invert the Nlevel formulas given in "Row Ranges"
above to round up to the end of a depth=2^k replication,
level = floor(log2(Ymax)) + 1
Nmax = 3^level - 1
For example Y=11, level=floor(log2(11))+1=4, so Nmax=3^4-1=80, which is the end of the
Y=15 row, ie. rounded up to the top of the replication block Y=8 to Y=15.
OEIS
The Sierpinski triangle is in Sloane's Online Encyclopedia of Integer Sequences in various
forms,
<http://oeis.org/A001316> (etc)
A001316 number of cells in each row (Gould's sequence)
A001317 rows encoded as numbers with bits 0,1
A006046 total cells to depth, being tree_depth_to_n(),
A074330 Nend, right hand end of each row (starting Y=1)
A001316 is the "rowpoints" described above. A006046 is the cumulative total of that
sequence which is the "Ndepth", and A074330 is 1 less for "Nend".
align="triangular" (the default)
A047999 0,1 cells by rows
A106344 0,1 cells by upwards sloping dX=3,dY=1
A130047 0,1 cells of half X<=0 by rows
A047999 etc is every second point in the default triangular lattice, or all points in
align="right" or "left".
align="triangular" (the default)
A002487 count points along dX=3,dY=1 slopes
is the Stern diatomic sequence
A106345 count points along dX=5,dY=1 slopes
dX=3,dY=1 sloping lines are equivalent to opposite-diagonals dX=-1,dY=1 in align="right".
align="right"
A075438 0,1 cells by rows including 0 blanks at left of pyramid
align="right", n_start=0
A006046 N on Y axis, being Ndepth
A074330 N on Diagonal starting from Y=1, being Nend
align="left", n_start=0
A006046 N on NW diagonal, being Ndepth
A074330 N on Y axis starting from Y=1, being Nend
A080263 Dyck encoding of the tree structure
A080264 same in binary
A080265 position in list of all balanced binary
A080268 Dyck encoding breadth-first
A080269 same in binary
A080270 position in list of all balanced binary
A080318 Dyck encoding breadth-first of branch-reduced
(duplicate each bit)
A080319 same in binary
A080320 position in list of all balanced binary
For the Dyck encoding see for example "Binary Trees" in Math::NumSeq::BalancedBinary. The
position in all balanced binary which is A080265 etc corresponds to "value_to_i()" in that
"NumSeq".
A branch-reduced tree has any single-child node collapsed out, so that all remaining nodes
are either a leaf node or have 2 (or more) children. The effect of this on the Sierpinski
triangle in breadth-first encoding is to duplicate each bit, so A080269 with each bit
repeated gives the branch-reduced A080319.
SEE ALSO
Math::PlanePath, Math::PlanePath::SierpinskiArrowhead,
Math::PlanePath::SierpinskiArrowheadCentres, Math::PlanePath::CellularRule,
Math::PlanePath::ToothpickUpist
Math::NumSeq::SternDiatomic, Math::NumSeq::BalancedBinary
HOME PAGE
<http://user42.tuxfamily.org/math-planepath/index.html>
LICENSE
Copyright 2011, 2012, 2013, 2014, 2015 Kevin Ryde
Math-PlanePath is free software; you can redistribute it and/or modify it under the terms
of the GNU General Public License as published by the Free Software Foundation; either
version 3, or (at your option) any later version.
Math-PlanePath is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with Math-
PlanePath. If not, see <http://www.gnu.org/licenses/>.