NAME
Math::NumSeq::Polygonal  polygonal numbers, triangular, square, pentagonal, etc
SYNOPSIS
use Math::NumSeq::Polygonal;
my $seq = Math::NumSeq::Polygonal>new (polygonal => 7);
my ($i, $value) = $seq>next;
DESCRIPTION
The sequence of polygonal numbers. The 3gonals are the triangular numbers i*(i+1)/2, the 4gonals are squares i*i, the 5gonals are pentagonals (3i1)*i/2, etc.
In general the kgonals for k>=3 are
P(i) = (k2)/2 * i*(i+1)  (k3)*i
The values are how many points are in a triangle, square, pentagon, hexagon, etc of side i. For example the triangular numbers,
d
c c d
b b c b c d
a a b a b c a b c d
i=1 i=2 i=3 i=4
value=1 value=3 value=6 value=10
Or the squares,
d d d d
c c c c c c d
b b b b c b b c d
a a b a b c a b c d
i=1 i=2 i=3 i=4
value=1 value=4 value=9 value=16
Or pentagons (which should be a pentagonal grid, so skewing a bit here),
d
d d
c d c d
c c d c c d
b c b c c b c d
b b b b c b b c d
a a b a b c a b c d
i=1 i=2 i=3 i=4
value=1 value=5 value=12 value=22
The letters "a", "b" "c" show the extra added onto the previous figure to grow its points. Each side except two are extended. In general the kgonals increment by k2 sides of i points, plus 1 at the end of the last side, so
P(i+1) = P(i) + (k2)*i + 1
Second Kind
Option pairs => 'second'
gives the polygonals of the second kind, which are the same formula but with a negative i.
S(i) = P(i) = (k2)/2 * i*(i1) + (k3)*i
The result is still positive values, bigger than the plain P(i). For example the pentagonals are 0,1,5,12,22,etc and the second pentagonals are 0,2,7,15,26,etc.
Both Kinds
pairs => 'both'
gives the firsts and seconds interleaved. P(0) and S(0) are both 0 and that value is given just once at i=0, so
0, P(1), S(1), P(2), S(2), P(3), S(3), ...
Average
Option pairs => 'average'
is the average of the first and second, which ends up being simply a multiple of the perfect squares,
A(i) = (P(i)+S(i))/2
= (k2)/2 * i*i
This is an integer if k is even, or k odd and i is even. If k and i both odd then it's an 0.5 fraction.
FUNCTIONS
See "FUNCTIONS" in Math::NumSeq for behaviour common to all sequence classes.
$seq = Math::NumSeq::Polygonal>new ()
$seq = Math::NumSeq::Polygonal>new (pairs => $str)

Create and return a new sequence object. The default is the polygonals of the "first" kind, or the
pairs
option (a string) can be"first" "second" "both" "average"
Random Access
$value = $seq>ith($i)

Return the
$i
'th polygonal value, of the givenpairs
type. $bool = $seq>pred($value)

Return true if
$value
is a polygonal number, of the givenpairs
type. $i = $seq>value_to_i_estimate($value)

Return an estimate of the i corresponding to
$value
.
SEE ALSO
Math::NumSeq, Math::NumSeq::Cubes
HOME PAGE
http://user42.tuxfamily.org/mathnumseq/index.html
LICENSE
Copyright 2010, 2011, 2012, 2013, 2014, 2016, 2018, 2019 Kevin Ryde
MathNumSeq 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.
MathNumSeq 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 MathNumSeq. If not, see <http://www.gnu.org/licenses/>.