Provided by: libmath-planepath-perl_113-1_all 
NAME
Math::PlanePath::PyramidSpiral -- integer points drawn around a pyramid
SYNOPSIS
use Math::PlanePath::PyramidSpiral;
my $path = Math::PlanePath::PyramidSpiral->new;
my ($x, $y) = $path->n_to_xy (123);
DESCRIPTION
This path makes a pyramid shaped spiral,
31 3
/ \
32 13 30 2
/ / \ \
33 14 3 12 29 1
/ / / \ \ \
34 15 4 1--2 11 28 ... <- Y=0
/ / / \ \ \
35 16 5--6--7--8--9-10 27 52 -1
/ / \ \
36 17-18-19-20-21-22-23-24-25-26 51 -2
/ \
37-38-39-40-41-42-43-44-45-46-47-48-49-50 -3
^
-6 -5 -4 -3 -2 -1 X=0 1 2 3 4 5 6 7
The perfect squares 1,4,9,16 fall one before the bottom left corner of each loop, and the pronic numbers
2,6,12,20,30,etc are the vertical upwards from X=1,Y=0.
Square Spiral
This spiral goes around at the same rate as the "SquareSpiral". It's as if two corners are cut off (like
the "DiamondSpiral") and two others extended (like the "OctagramSpiral"). The net effect is the same
looping rate but the points pushed around a bit.
Taking points up to a perfect square shows the similarity. The two triangular cut-off corners marked by
"."s are matched by the two triangular extensions.
+--------------------+ 7x7 square
| . . . 31 . . .|
| . . 32 13 30 . .|
| . 33 14 3 12 29 .|
|34 15 4 1 2 11 28|
35|16 5 6 7 8 9 10|27
36 17|18 19 20 21 22 23 24|25 26
37 38 39|40 41 42 43 44 45 46|47 48 49
+--------------------+
N Start
The default is to number points starting N=1 as shown above. An optional "n_start" can give a different
start, with the same shape etc. For example to start at 0,
12 n_start => 0
/ \
13 2 11
/ / \ \
14 3 0--1 10
/ / \
15 4--5--6--7--8--9
/
16-17-18-19-20-21-22-...
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
"$path = Math::PlanePath::PyramidSpiral->new ()"
"$path = Math::PlanePath::PyramidSpiral->new (n_start => $n)"
Create and return a new pyramid spiral object.
"$n = $path->xy_to_n ($x,$y)"
Return the point number for coordinates "$x,$y". $x and $y are each rounded to the nearest integer,
which has the effect of treating each N in the path as centred in a square of side 1, so the entire
plane is covered.
OEIS
This path is in Sloane's Online Encyclopedia of Integer Sequences as
<http://oeis.org/A053615> (etc)
n_start=1 (the default)
A053615 abs(X), distance to next pronic, but starts n=0
A054552 N on X axis, 4n^2 - 3n + 1
A033951 N on South-East diagonal, 4n^2 + 3n + 1
A214250 sum N of eight surrounding cells
A217013 permutation N of points in SquareSpiral order
rotated +90 degrees
A217294 inverse
In the two permutations the pyramid spiral is conceived as starting to the left and the square spiral
starting upwards. The paths here start in the same direction (both to the right), hence rotate 90 to
adjust the orientation.
n_start=0
A001107 N on X axis, decagonal numbers
A002939 N on Y axis
A033991 N on X negative axis
A002943 N on Y negative axis
A007742 N on diagonal South-West
A033954 N on diagonal South-East, decagonal second kind
n_start=2
A185669 N on diagonal South-East
SEE ALSO
Math::PlanePath, Math::PlanePath::SquareSpiral, Math::PlanePath::PyramidRows,
Math::PlanePath::TriangleSpiral, Math::PlanePath::TriangleSpiralSkewed
HOME PAGE
<http://user42.tuxfamily.org/math-planepath/index.html>
LICENSE
Copyright 2010, 2011, 2012, 2013 Kevin Ryde
This file is part of Math-PlanePath.
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/>.