Prime Numbers

< 8.5 Number Theory | Topic Index | 8.7 Fundamental Theorem of Arithmetic >

We have seen that whole numbers which cannot be divided by any number, other than 1 and the number itself, are called Prime Numbers. For example, 7 is a prime as its factors are only 1 &amp; 7. 6 is not prime since its factors are 1, 2, 3 &amp; 6.

It was discovered that any whole number can be written as a product of powers of prime numbers &ldquo;in an unique way&rdquo;. This is called the Fundamental Theorem of Arithmetic which we will see in detail in the next chapter. For this reason, prime Numbers are considered the building blocks of all numbers.

Sieve of Eratosthenes

Prime numbers have been studied right from early days of mathematics. Eratosthenes, a Greek mathematician who lived in the 3rd century BCE, evolved a systematic way of locating all prime numbers up to any desired number.

You start with a grid of numbers from 1 to the desired number. The user systematically crosses out numbers which can be divided by prime numbers starting with 2, 3, 5 and so on. Ultimately only the prime numbers would be left uncrossed.

For obvious reasons, it was called the Sieve of Eratosthenes.

A lot of higher mathematics is based on primes. There are many theorems in Number Theory which involve Prime numbers. We will now see a few interesting facts about them.

'1 is not prime but 2 is a prime'

There are reasons in mathematics why 1 is not considered a prime but 2 is. This is due to the basic theorem of arithmetic (refer to Chapter 8.7) which states that every number can be represented as a product of powers of prime numbers in an unique manner.

1 is a factor of every number and product of any number of 1s has the same value. So if 1 is taken as a prime, there cannot be an unique representation for any number. 6 can be written as 2 & 3 or 1, 2 & 3 or 1, 1, 2 & 3!. Hence mathematicians have decided that 1 will not be considered a prime.

Every alternate whole number is an even number which has 2 as a factor. So for the sake of unique representation, 2 is considered as a prime number. So 2 becomes the only even prime!

Goldbach&rsquo;s Conjecture -

In 1742, Christian Goldbach made the conjecture that &ldquo;any even number can be written as the sum of 2 prime numbers&rdquo;. Though the above statement has not been proved mathematically to be correct, until today, even after analysing millions of numbers, no exception to the rule has been found.

This is also an example of Number Theory which we dealt with in the previous chapter.

Infinite number of primes

Euclid gave a very elegant proof that there are an infinite number of primes. His proof showed that with the known primes, we can always construct another bigger prime.

Further, no detectable pattern has been seen in the sequence of primes.

Use of prime numbers in cryptography

Until 1970s prime numbers were studied only as exercises in mathematical logic.

With the development of computers &amp; telecommunications vast amounts of financial and confidential data were being sent through them. Such data could easily be accessed by persons who know the technical workings of a computer network. Hence the issue of safety of the data became critical. One solution was to &lsquo;code&rsquo; the data using various techniques so that only those who knew the code could read them.

One simple technique could be to change a word like &ldquo;Money&rdquo; into another word &ldquo;Nlmvb&rdquo;. Here each letter has been replaced by its counterpart reading the alphabet in the reverse direction. &ldquo;y&rdquo; is the 25thletter (2ndfrom last) and it is replaced by &ldquo;b&rdquo; (2ndfrom the first). This is called the &ldquo;key&rdquo; to the coding. Unless some one knows the key, she cannot decipher the original word.

These techniques were needed badly during World War II and developed into the mathematical science of Cryptography. Invention of computer software accelerated the development.

With the spread of digital transfers of data there rose a need to encrypt the data for secrecy. Many sophisticated systems of encrypting were invented. One of them was the use the product of 2 very large (with hundreds of digits) prime numbers to generate the keys required to decrypt the data. The safety of this method rests on the fact that factorizing the product of such huge prime numbers was almost impossible even for superfast computers.

Largest identified prime

The use of primes in cryptography motivated the discovery of larger and larger primes. Faster computers were used for this. Even with a computer, the larger the number, more difficult the process to check if it is a prime. Hence faster algorithms and computer programs were also developed to check if a number is prime.

About 30 years back mathematicians have started an ongoing project called the Great Internet Mersenne Prime Search, basically to discover larger & larger primes.

The largest known prime number discovered, as of July 2018, is 2 77,232,917  &minus; 1, a number with 23,249,425 digits.

Primes in the Search for Intelligent Life in the Universe

Scientists have made a reasonable guess that if there are other intelligent beings in the Universe, they would have discovered the basics of mathematics and would be aware of prime numbers. Hence one idea of discovering any intelligent life is to send a message containing the sequence of primes in terms of simple dots & dashes.

The first such effort was made in 1967 when the Arecibo radio telescope in Puerto Rico was inaugurated. A message of length 1679 was sent to a star cluster about 25,000 light years away. No reply has been received until now.

< 8.5 Number Theory | Topic Index | 8.7 Fundamental Theorem of Arithmetic >