number theory

Amicable Numbers

Amicable numbers are pairs of positive integers where each number equals the sum of the proper divisors of the other number. Proper divisors are all the positive divisors of a number except the number itself.

The smallest pair of amicable numbers is 220 and 284:

amicable numbers explanation

Amicable numbers may not appear in everyday calculations, but they play an important role in mathematics. They help us understand how numbers relate through their factors and divisors, sharpen pattern recognition, and build strong logical thinking. Often used in number theory, teaching, and programming practice, amicable numbers remind us that some parts of mathematics exist not for direct application, but to train the mind to think clearly and deeply.

Math titbits

For example:

If we consider the three consecutive numbers 7,8 and 9

82 = 64

7 X 9 = 63.

Kaprekar constant

The number 6174 is called the Kaprekar constant. It was discovered by the Indian Mathematician Kaprekar.

We get this constant if we perform certain routine calculations to any four-digit number.

The rule is

1. Take any 4 digit number.

2. Arrange the digits of the 4 digit number in descending and ascending order.

3. Subtract the smaller number from the bigger number

4. Repeat the steps from 2 for the answer obtained also.

Repeat the steps till you reach 6174.

We always end up with 6174. Once you reach 6174, the process will continue generating the same number.

Let us solve an example to understand this..

Kaprekar constant

Kaprekar Number

A Kaprekar number is

A number’s square divided into two parts such that the sum of its parts is equal to the original number.

Kaprekar Number

An example of this Kaprekar Number is explained in the following video.

Bertrand’s Postulate

For every number greater than 1, there is at least one prime number between the number and its double.

Bertrand’s postulate states that there is at least one prime number p, such that n < p < 2n.

Pictorial representation

This Postulate was first proposed by Bertrand in 1845.

However, Bertrand did check that his conjecture was true up to 3 million.

It was first proved in 1850 by Chebyshev so is also called the Bertrand-Chebyshev Theorem.

Chebyshev on a 2021 stamp of Russia

The Indian Mathematician Ramanujan, who was not aware of Chebyshev’s proof, came up with an easier proof on 1919.

In 1932, a Hungarian Mathematician, Paul Erdos, came with another different proof.

Goldbach’s Conjecture

Goldbach’s conjecture is one of the oldest unsolved problems in mathematics.


Any even number greater than 2 can be written as the sum of two prime number.

Every odd whole number greater than 5 can be written as the sum of three primes. 

Goldbach’s conjectures

Goldbach, a Prussian mathematician(1690-1764)

On 7 June 1742, Goldbach wrote a letter to the mathematician Euler in which he proposed the conjectures he found.

Euler responded back on 30 June 1742 as

Every integer greater than 2 is a sum of two primes, I regard this as a completely certain theorem, although I cannot prove it.

Euler

Though the Goldbach’s conjecture looks simple to understand, it has not yet been proved.

In the modern times, the conjectures proposed by Goldbach is identified as weak or strong Goldbach conjecture.

STRONG GOLDBACH CONJECTURE

Any even whole number greater than 2 can be written as the sum of two prime number.

WEAK GOLDBACH CONJECTURE

Every odd whole number greater than 5 can be written as the sum of three primes. 

In 2013, Harold Helfgott, a Peruvian mathematician released two papers claiming the proof of Goldbach weak conjecture and The proof was accepted for publication in the Annals of mathematics series in 2015, and has been undergoing further review and revisions.

In March 2000 the publishers of the book ‘Uncle Petros and Goldbach’s conjecture (Bloomberg in the USA and Faber and Faber in the UK) offered a prize of one million dollars for anyone who could prove the Goldbach’s conjecture

Any even whole number greater than 2 can be written as the sum of two prime number.

The prize was kept open for two years, but nobody claimed it.

Odd(number) Fact

A cool fact about odd numbers.

2520 – Amazing fact about this number.

2520

Dudeney Number

Henry Dudeney, a self taught Mathematician known for his puzzles, first noted the existence of these numbers while framing one of his puzzles.

What’s so special about these numbers?

What is the pattern followed behind the construction of these numbers?

Yes! There is a very interesting pattern behind these numbers.

Let’s check that pattern in the following video.

Dudeney numbers

The magic number 1089

This math trick helps the kids think about the patterns behind the calculation and their amazing fixed answer.

Mastering this concept will make them believe that they are a tricky mathe-magician.

The following video will explain about the math trick behind the magic number 1089.

The mathematical proof of the above trick is as follows: