Testing for primality

How to test a huge number for primality? Provides an implementation of the famous Miller-Rabin algorithm using the widely available ‘bc’ (basic calculator) tool. Suitable both for Linux and Windows.

Testing if a number is prime is an interesting problem. Prime numbers have uses in many fields especially Cryptography. Here we show a technique through which we can verify if a number is prime or composite (not with 100% guarantee though). The algorithm we use is called Miller-Rabin.

I have, in another blogpost, talked about a tool available on Unix called bc which stands for basic calculator. In truth, its not ‘basic’ by any stretch of imagination. For my Windows brethren, a version is part of UnxUtils. This tool allows you to do large number, arbitrary precision calculations. It also allows you to write scripts. The script to implement this test is called pcheck.bc.

On Unix, it can be run using ./pcheck.bc while on windows it needs to be run like this:

bc pcheck.bc -ql.

The '-l' option is for bc to load math. library, and the '-q' option is to stop bc from boasting when it runs.

First, it will ask you for an option. The algorithm runs in iterations, each iteration requiring the given number (called p) to be tested against another number from a set (called s). If p is composite against s, then p is composite else p is probably prime. In order to increase this probability, this implementation tests against 30 given numbers. If option is entered as ‘0’, the program will read the numbers in set from the input and you can enter the numbers suitably. If its entered as ‘1’, the program will automatically select the set. If you are able to enter random numbers then select option ‘0’ else select ‘1’. I may talk about generating arbitrary long random numbers in a future blogpost.

Second, it will ask you for the number to test. Enter any large odd number (even numbers are composite). Sample:

900900900900990990990991 (prime)
900900900900990990990993 (composite)
359334085968622831041960188598043661065388726959079839 (prime)

Third, if you selected option ‘0’, it will prompt you for the 30 random numbers.

Read the code to understand further – its not very difficult to understand. Have questions, post comments.

This can be further extended to search for primes: start at any big odd number, test it – it its prime report and stop, if not add 2 to the number and test again.


Value of Pi


Wikipedia defines Pi or p as “a mathematical constant which represents the ratio of any circle’s circumference to its diameter in Euclidean geometry”. As a kid I used to be interested in the calculation of Pi. The value of Pi, to 100 places of decimal is:

3.1415 9265 3589 7932 3846 2643 3832 7950 2884 1971 6939 9375 1058 2097 4944 5923 0781 6406 2862 0899 8628 0348 2534 2117 0664

Value to 100 places
Value to 100 places

I have calculated this using the Unix command bc. The command for this is based on an identity (that I think is credited to Ramanujan) and is:
where a stands for the arctangent function.

The formula is:

pi = 24*arctan(1/8)+8*arctan(1/57)+4*arctan(1/239)

First, load the bc language with associated library using “bc -l”. Then, set the scale to the number of digits using “scale=100”. Afterwards run the identity I gave above.

To experimentally calculate Pi experimentally, there are two ways: One using a random number generator and the other by physically measuring the circumference of a given circle.

Consider a square of length unity. Within that, a circle is drawn, having unity diameter. If a point is taking within the square at random, the probability of that point also lying within the circle is Pi*(1/2)*(1/2) which is Pi/4. Now, start taking points at random (x,y) and see if x*x+y*y<=1/4 or not (if it is, then that means the point lies within the circle). Maintain the count of total number of points taken (t), and the number that fell within the circle(c). Now Pi can be calculated as:


The other method is to take a circular bottle (measure the radius r) or tin and tie a thread around its circumference. Measure the circumference(c). Now Pi is c/2r.

The following two sentences contain the value of Pi: the number of letters in each word indicates the corresponding number in the value of Pi.

1. “May I have a large cup of coffee.”

2. “How I want a drink, alcoholic of course, after the heavy chapters involving quantum mechanics.”

This makes the value of Pi easy to remember.

Lastly, how useful are the digits of Pi as a source of random numbers? Not bad, according to a study: “while sequences of digits from pi are indeed an acceptable source of randomness – often an important factor in data encryption and in solving certain physics problems – pi’s digit string does not always produce randomness as effectively as manufactured generators do”.

Always wanting to do my own thing, I downloaded the value of Pi to one million places from a website, split it into (x,y,z) coordinates, each having 5 digit precision. Here is the graph that got generated:


A bell curve, but could have been better – seems to mimic the results from the study.