Monte Carlo Pi Estimate

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Problem Description

The other day someone told me that the probability that any randomly chosen pairs of integers are relatively prime is an interesting number.

"In fact," she said, " if we divide that probability into 6 and take the square root, the result is Pi!" 

This program  checks and see if that could be true.

Numbers are relatively prime if they have no common factors greater than 1. Pi is the ratio of the circumference of a circle to its diameter.

Background & Techniques

I can't remember where I ran across this hypothesis.   Nor have I been able to find the math, or even  a rationale as to why it might be true.   But it does appear to be. 

We'll get a number of random pairs to check from the user and then loop that many times counting the number that are relatively prime.  At the end of the loop, the Pi estimate is calculated  as the SQRT(6*NbrPairs / NbrPairsRelativelyPrime)

Saying that two numbers are relatively prime is the same as saying that their greatest common divisor is 1.  We'll use a simple GCD function to check for relative primeness.    For more info you can check  this GCD Math Topic

The usual features of my programs with potentially long  loops are here:

  1. A STOP button to end the loop.   Positioned over the Start button and defined initially as invisible.
  2. Before starting of the loop, make the STOP button visible, set Form1.Tag:=0 and  Screen.Cursor:=crHourGlass.
  3. When the STOP button is pressed, set Tag:=1;
  4. Inside the loop, check for Tag<>0 periodically (every 1024 passes in this program) and break out if true.
  5. After the loop ends make he STOP button invisible again and set the cursor back to crDefault.

And in case you want to try several million samples, results are accumulated and the cumulative  estimate also displayed with each run.

Running/Exploring the Program 

bulletBrowse source extract
bulletDownload source
bulletDownload  executable

Suggestions for Further Explorations

Check out series summing methods which give much better estimates much faster than random sampling methods.   The best of the known algorithms for calculating Pi is the Borwein quartically convergent algorithm and has been used to calculate the first 200 billion digits of Pi.   Each iteration adds several hundred digits of accuracy.

The whole "Pi digits" exercise may be mathematically pleasing but probably has little real world application.  The first 39 digits of PI were  calculated  350 years ago.  That is accurate enough to calculate the circumference of a circle 40 billion light years in diameter with a error of less than the width of one hydrogen atom!  I'd say that 39 digits is sufficient for all practical purposes.   

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