NAME

Math::PlanePath::SacksSpiral -- circular spiral squaring each revolution

SYNOPSIS

 use Math::PlanePath::SacksSpiral;
 my $path = Math::PlanePath::SacksSpiral->new;
 my ($x, $y) = $path->n_to_xy (123);

DESCRIPTION

The Sacks spiral by Robert Sacks is an Archimedean spiral with points N placed on the spiral so the perfect squares fall on a line going to the right. Read more at

<http://www.numberspiral.com>

An Archimedean spiral means radial distance a constant factor of the angle, and so each loop a constant distance out from the preceding loop, in this case 1 unit out. The polar coordinates are

    R = sqrt(N)
    theta = sqrt(N) * 2pi

which comes out roughly as

                    18
          19   11        10  17
                     5
             
    20  12  6   2
                   0  1   4   9  16  25
                   3
      21   13   7        8
                             15   24
                    14
               22        23

The X,Y positions returned are fractional, except for the perfect squares on the positive X axis at X=0,1,2,3,etc. The perfect squares are the closest points, at 1 unit apart. Other points are a little further apart.

The arms going to the right like N=5,10,17,etc or N=8,15,24,etc are constant offsets from the perfect squares, ie. d^2 + c for positive or negative integer c. To the left the central arm N=2,6,12,20,etc is the pronic numbers d^2 + d = d*(d+1), half way between the successive perfect squares. Other arms going to the left are offsets from that, ie. d*(d+1) + c for integer c.

Euler's quadratic d^2+d+41 is one such arm going left. Low values loop around a few times before straightening out at about y=-127. This quadratic has relatively many primes and in a plot of primes on the spiral it can be seen standing out from its surrounds.

Plotting various quadratic sequences of points can form attractive patterns. For example the triangular numbers k*(k+1)/2 come out as spiral arcs going clockwise and anti-clockwise.

See examples/sacks-xpm.pl for a complete program plotting the spiral points to an XPM image.

FUNCTIONS

See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.

"$path = Math::PlanePath::SacksSpiral->new ()"

Create and return a new path object.

"($x,$y) = $path->n_to_xy ($n)"

Return the X,Y coordinates of point number $n on the path.

$n can be any value "$n >= 0" and fractions give positions on the spiral in between the integer points.

For "$n < 0" the return is an empty list, it being considered there are no negative points in the spiral.

"$rsquared = $path->n_to_rsquared ($n)"

Return the radial distance R^2 of point $n, or "undef" if there's no point $n. This is simply $n itself, since R=sqrt(N).

"$n = $path->xy_to_n ($x,$y)"

Return an integer point number for coordinates "$x,$y". Each integer N is considered the centre of a circle of diameter 1 and an "$x,$y" within that circle returns N.

The unit spacing of the spiral means those circles don't overlap, but they also don't cover the plane and if "$x,$y" is not within one then the return is "undef".

Descriptive Methods

"$dx = $path->dx_minimum()"

"$dx = $path->dx_maximum()"

"$dy = $path->dy_minimum()"

"$dy = $path->dy_maximum()"

dX and dY have minimum -pi=-3.14159 and maximum pi=3.14159. The loop beginning at N=2^k is approximately a polygon of 2k+1 many sides and radius R=k. Each side is therefore

    side = sin(2pi/(2k+1)) * k
        -> 2pi/(2k+1) * k
        -> pi

"$str = $path->figure ()"

Return "circle".

FORMULAS

Rectangle to N Range

R=sqrt(N) here is the same as in the "TheodorusSpiral" and the code is shared here. See "Rectangle to N Range" in Math::PlanePath::TheodorusSpiral.

The accuracy could be improved here by taking into account the polar angle of the corners which are candidates for the maximum radius. On the X axis the stripes of N are from X-0.5 to X+0.5, but up on the Y axis it's 0.25 further out at Y-0.25 to Y+0.75. The stripe the corner falls in can thus be biased by theta expressed as a fraction 0 to 1 around the plane.

An exact theta 0 to 1 would require an arctan, but approximations 0, 0.25, 0.5, 0.75 from the quadrants, or eighths of the plane by X>Y etc diagonals. As noted for the Theodorus spiral the over-estimate from ignoring the angle is at worst R many points, which corresponds to a full loop here. Using the angle would reduce that to 1/4 or 1/8 etc of a loop.

HOME PAGE

<http://user42.tuxfamily.org/math-planepath/index.html>

LICENSE

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/>.

2021-01-23

perl v5.32.0