# π Places

π (say ‘pie’) is the lower case Greek letter symbol that stands for a number defined as the ratio of the circumference (C) of a circle divided by its diameter (d).

Circumference means the distance around something, like a circle or a polygon. A synonym would be perimeter.

A diameter is the length of a line that is drawn starting from any point on the circle’s boundary and going through the center point of the circle and then continuing to the opposite boundary point. A radius (r) is the length of a line drawn from the center to any point on the circle’s boundary. It is half the diameter. It’s clearer in the diagram:

As a formula (no numbers, please), we write:

pi = C/d                                                   [1]

If d = 1 inch, then because π has a fixed, constant value, C must be equal to that value of π inches.

A closely related formula involving π involves the Area A, of a Circle and its radius r, (remember? – half of the diameter’s length). The formula is:

pi = A/r^2                                                   [2]

Most descriptions of π state when it was first historically used and trace a series of numerical refinements in its approximated value over the last 4500 years.

A chronology of the computation of π from 2500 BC (calculated as 22/7 ) to the present is shown in the following link: π Chronology

It shows the date, the person, milestone or event and the decimal value of π (or the correct number of decimal digits after 3.) up through October 8, 2014, when π had been computed and verified (anonymously) to 1.33∙10^13 (over 13 Trillion) decimal digits.

## Classifications

It turns out that π is an irrational number. (Johann Heinrich Lambert proved this in 1761.)

This means that we cannot discover a fraction that provides its value exactly. Writing its decimal digits never ends. So over the years, mathematicians have labored to compute π accurately to more and more decimal places using “good” fractions and infinite series where only the first “few” terms are evaluated and summed.

It is also true the π is a transcendental number. A transcendental number is an irrational number that is not an algebraic number. An algebraic number is one that satisfies a polynomial equation with a finite number of terms and with rational coefficients. So, for example, sqrt(2) is an irrational (non-repeating decimal number) and an algebraic number, since it is a solution to the algebraic equation: x^2 = 2

For non-algebraic equations, we have two equations from trigonometry, which use pi/4 radians rather than 45 degrees:

tangent(pi/4) = 1                                                  [3]

and particularly its counterpart:

arctangent(1) = pi/4                                                [4]

is used as a starting point for determining π more precisely.

## Rational (Fractional) Approximations

Methods of approximation started with Archimedes, who, around 250 BC, applied the method of exhaustion with inscribed and circumscribed regular polygons to enclose the lower and upper bounds of area and circumference of the circle. Shown below are inscribed and circumscribed pentagons, hexagons and octagons (n = 5, 6, 8). Starting with a hexagon, he doubled the sides for inner and outer n-gons until n = 96, finding the inner and outer perimeters at each value of n.

Archimedes found bounds for π as:

3 + 10/71 < pi < 3 + 10/70      or      223/71 < pi < 22/7                   [5]

This gives the value of π to 2 decimal places, as 3.14.

### A Cool Procedure And Shell Script For a better Fraction for π

Using the circumference formula [1], we can create a procedure based on one suggested by Rhett Allain (See:
Best Fraction For π to calculate a fraction that represents π for a given number of digits.

For reference, suppose πref is set to 3.14159265358979, which is to 14 decimal places. We use this 12 step procedure:

1. Set πref = 3.14159265358979
2. Set (1/πref) = 0.31830988618379
3. Set loopcount = 500
4. Set C = 22
5. Set D = 7
6. Set Percentage Error = 0.001
7. Is loopcount = 0 ?
Then output BestC, BestD, BestC/BestD, Error, BestLoop
and stop

8. Set πest = C/D
9. Calculate Absolute Value of E = | (πref – πest) / πref |
via E = | (πref – πest) ∙ (1/πref) |
10. Is E < Error ?
Then set BestC = C, set BestD=D, set Error = E,
and set Bestloop = 500 – loopcount

11. Is πest < πref ?
Then add 1 to C and go to step 12
Otherwise add 1 to D and go to step 12

12. Subtract 1 from loopcount and go back to step 7 and compute new πest

Here is the bash shell script that I wrote to program this algorithm. It takes slightly under 7.5 seconds (what’s your hurry?) to finish 500 iterations (on a 2010 vintage 32-bit MacBook Pro) and 3.55 seconds on a 64-bit GNU/Linux Server.

The script successfully finds the fraction

355/113 = 3.141592920353982                                              [6]

a close approximation to the reference π that is accurate to 6 decimal places. The percentage error using this fraction is 0.0000084913679%

In this script, the Linux utility bc is needed to offer/preserve 15 decimal place precision in the computations it performs. The shell facilities for operating with numbers using anything other than positive integers are minimal.


#! /bin/bash
USAGE="Usage: pifraction.bash"
# Version 1.0 by arkay on 1/17/2015
# Version 1.1 by arkay on 1/18/2015
#   Computed (1/Pi_ref) once and used it to multiply for
#   % difference.

# Set Starting values for variables
Pi_ref="3.14159265358979"
Pi_ref_inv="0.31830988618379"
loopcount=500
C=22
D=7
Error="0.001"
# loop 500 times
until (( loopcount < 1 ))
do
# Calculate Pi.est
Pi_est=$(echo "scale=15;$C/$D" | bc -l) # Calculate the percentage error E E=$(echo "scale=15; ($Pi_ref -$Pi_est)*$Pi_ref_inv" | bc -l) if [ "${E%%[0-9.]*}" == "-" ]  #Apply Absolute Value:
# extract first character of E, either "–" or ""
then
E=$(echo "scale=15; (-1)*$E" | bc -l)
fi
# Compare E with current minimum % Error
T=$( echo "scale=15;$E<$Error" | bc -l ) if [ "$T" -eq "1" ] # bc returns 1 if inequality true
then
BestC=$C; BestD=$D; Error=$E; BestLoop=$(expr 500 - $loopcount) fi # Compare Pi_est with Pi_ref S=$( echo "scale=15; $Pi_est<$Pi_ref" | bc -l )
if [ "$S" -eq "1" ] # bc returns 1 if inequality true then (( C += 1)) else (( D += 1)) fi (( loopcount -= 1 )) done # Produce the results BestCoverD=$(echo "scale=15; $BestC/$BestD" | bc -l)
echo "BestC=$BestC BestD=$BestD BestCoverD=$BestCoverD" echo "Pi_ref=$Pi_ref  Error=$Error BestLoop=$BestLoop"
exit   # Normal stopping point

# End of pifraction.bash



Output results from this shell script are shown as:
BestC=355 BestD=113 BestCoverD=3.141592920353982
Pi_ref=3.14159265358979 Error=.000000084913679 BestLoop=439

In my next post about π, I will write about greater decimal place approximators and Interesting π activities to try.

Oh, one last thing…
This is a delightful Math Trick attributed to Martin Gardner:

Write all 26 letters of the alphabet, but start with the letter J as shown:

JKLMNOPQRSTUVWXYZABCDEFGHI

Then, remove all the letters that have vertical symmetry as shown:

JKL   N   PQRS              Z   BCDEFG

Now, count the letters that remain in each subset: 3 1 4 1 6.

# Factorials For Fun

Start from the beginning. A factorial is a specific multiplication function applied to a positive integer value and all of its positive descendants. True but vague and tough to digest.

A factorial means to multiply a consecutive sequence of descending natural numbers.

Below, n is any positive integer (equal to a natural number), being multiplied:

n! = n·(n–1)·(n–2)·…·3·2·1                                                                                             [1]

The symbol for a factorial is the exclamation point: ! . n! is pronounced ‘en’ factorial (math fans) or ‘en’ bang (programmers) or shriek out n (literalists and comedians).

Here are some examples:                      3! = 3·2·1 = 6

10! = 10·9·8·7·6·5·4·3·2·1 = 3628800

{ Sudden question: In 1 second, how much would  9!  be? }

1! = 1

To see the first 100 factorials, please admire the following table by N. J. A. Sloane:   First 100 Factorials

These integer results become huge as n increases. In a future post, I will write about how to determine the number of digits of n! without calculating the factorial and visually counting the digits.

Because mathematicians revere economy in writing, n! can be defined for any positive integer n by two rules:

n! = n·(n–1)!
and                                                                                                                              [2]
1! = 1

meaning, n! is the product of n and the factorial of the next lower value shown as (n – 1).

In the case of the second rule, this only works if we also have

0! = 1                                                                                                               [3]

Since by rule 1 (and I know this may be a head-scratcher…)

1! = 1·0! = 1                                                                                                      [4]

To find the value of n! , the first rule is applied repeatedly, as shown:

n! = n·(n–1)! = n·(n–1)·(n-2)! = … =
n
·(n–1)·(n-2)··3·2·1! =                                                                                  [5]
n
·(n–1)·(n-2)··3·2·1

At this point, the original definition of n! in [1] has been recreated.

This [2] is called a recursive definition of n! . It has been said anonymously that, “in order to understand recursion, one must first understand recursion”.

In Computer Programming, n! can be determined via an iterative set of instructions, involving looping. Also, n! can be determined via a recursively called function or program, which calls itself repeatedly.

The Rosetta Code.org Wiki for factorial lists approximately 179 different programming languages with both iterative and recursive factorial code segments. It’s quite impressive – See:
Factorial Programs and Functions – Rosetta Code

Within this listing are programs that are part of the Unix/Linux facilities: awk, bash, bc, dc, JavaScript, m4, perl, php, python, R, ruby, and Unix shells (sh, ksh, zsh). These programs do not require a compiler. In future posts, these will be examined more closely.

There’s much more to factorials, which I will defer to further posts .

It has been suspected that mathematicians invented the factorial symbol (!) as a way of making mathematics look more exciting; after all, as the value of n increases, its factorial increases incredibly fast.

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