/ 
Math-PlanePath-5
River stage one • 3 direct dependents • 4 total dependents
/ Math::PlanePath::PyramidRows

NAME

Math::PlanePath::PyramidRows -- points stacked up in a pyramid

SYNOPSIS

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

DESCRIPTION

This path arranges points in successively wider rows going upwards so as to form an upside-down pyramid. The default step is 2, ie. each row 2 wider than the preceding, one square each side,

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

-4  -3  -2  -1  x=0  1   2   3   4 ...

The right end here 1,4,9,16,etc is the perfect squares. The vertical 2,6,12,20,etc at x=-1 is the pronic numbers s*(s+1), half way between those successive squares.

The step 2 is the same as the PyramidSides, Corner and SacksSpiral paths. For the SacksSpiral, spiral arms going to the right correspond to diagonals in the pyramid, and arms to the left correspond to verticals.

Step Parameter

A step parameter controls how much wider each row is than the preceding, to make wider pyramids. For example step 4

my $path = Math::PlanePath::PyramidRows->new (step => 4);

makes each row 2 wider on each side successively

29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45        4
      16 17 18 19 20 21 22 23 24 25 26 27 28              3
             7  8  9 10 11 12 13 14 15                    2
                   2  3  4  5  6                          1
                         1                          <-  y=0

      -6 -5 -4 -3 -2 -1 x=0 1  2  3  4  5  6 ...

If the step is an odd number then the extra is at the right, so step 3 gives

13  14  15  16  17  18  19  20  21  22        3
     6   7   8   9  10  11  12                2
         2   3   4   5                        1
             1                          <-  y=0

-3  -2  -1  x=0  1   2   3   4 ...

Or step 1 goes solely to the right. This is equivalent to the Diagonals path, but columns shifted up to make horizontal rows.

11  12  13  14  15
 7   8   9  10                    3
 4   5   6                        2
 2   3                            1
 1                          <-  y=0

x=0  1   2   3   4 ...

Step 0 means simply a vertical, each row 1 wide and not increasing. This is unlikely to be much use. The Rows path with width 1 does this too.

 5        4
 4        3
 3        2
 2        1
 1    <-y=0

x=0

Various number sequences fall in regular patterns positions depending on the step. Large steps are not particularly interesting and quickly become very wide. A limit might be desirable in a user interface, but there's no limit in the code as such.

Step 3 Pentagonals

For step 3 the pentagonal numbers 1,5,12,22,etc, (3k-1)*k/2, are at the rightmost end of each row. The "second pentagonal numbers" 2,7,15,26, S(k) = (3k+1)*k/2 are the vertical at x=-1. (Those second numbers are obtained by taking negative k in the plain pentagonal (3k-1)*k/2, and the two together are the "generalized pentagonal numbers".)

The second pentagonals are not prime beyond 7 since they're (3k+1)*k/2 with the denominator 2 dividing into one or the other part. Numbers S(k)-1 immediately to the left are never prime either since S(k)-1 = (3*k-2)(k+1)/2. Likewise S(k)-2 = (3*k+4)(k-1)/2 beyond 13. If you plot the primes on a step 3 PyramidRows then these sequences make a 3-wide vertical gap at x=-1,-2,-3 where there's no primes.

          no primes in these three columns
          except the low values 2,7,13
          |  |  |
52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70
   36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51
      23 24 25 26 27 28 29 30 31 32 33 34 35
         13 14 15 16 17 18 19 20 21 22
             6  7  8  9 10 11 12
                2  3  4  5
                   1
-6 -5 -4 -3 -2 -1 x=0 1  2  3  4 ...

In general a constant offset c from S(k) is a column and the simple factorization above using the roots of the quadratic S(k)-c = (3/2)*k^2 + (1/2)*k - c is possible whenever 24*c+1 is a perfect square. This means the further columns S(k)-5, S(k)-7, S(k)-12, etc have no primes either. The S(k), S(k)-1, S(k)-2 ones are the most prominent because they're adjacent. There's no other adjacent columns of this type because the squares become too far apart for successive 24*c+1. Of course there could be many other reasons for other columns to be prime-free or nearly so or above a certain point, etc.

FUNCTIONS

$path = Math::PlanePath::PyramidRows->new ()
$path = Math::PlanePath::PyramidRows->new (step => $s)

Create and return a new path object. The default step is 2.

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

Return the x,y coordinates of point number $n on the path.

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

$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 point in the pyramid as a square of side 1. If $x,$y is outside the pyramid the return is undef.

SEE ALSO

Math::PlanePath, Math::PlanePath::PyramidSides, Math::PlanePath::Corner, Math::PlanePath::SacksSpiral

Math::PlanePath::Diagonals, Math::PlanePath::Rows

HOME PAGE

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

LICENSE

Math-PlanePath is Copyright 2010 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/>.