Hello dear readers. Book page number, bank account, day of the week, time, addresses, etc. – it all consists of numbers. But what do we really know about them? In fact, they are more fun than it seems at first glance. And so today we decided to better acquaint you with this basic concept of mathematics, used for quantitative characteristics, comparison, numbering of objects and their parts.

## # 1

There are an infinite number of primes – as proved by the ancient Greek mathematician Euclid and a number of modern mathematicians. But only two prime numbers 2 and 5 end in “2” and “5”. The rest end with “1”, “3”, “7”, “9”.

## # 2

Many people mistakenly think that 1 is a prime number. In fact, 1 is neither simple nor composite, since it has only 1 divisor – 1. As for the smallest prime, it is 2.

## No. 3

Ancient civilizations knew the value of π to two decimal places, and by the fifth century, Chinese mathematicians brought it closer to seven digits. The number π is irrational, that is, it cannot be represented as a fraction (as the ratio of two integers), and its decimal representation never ends.

## No. 4

The angles of a triangle always add up to 180 degrees. There are several types of triangles, but the sum of all angles will always be 180 degrees.

## No. 5

There are only 2 numbers, the number of letters in which coincides with their numerical value. These are “3” and “11”. The word “three” has three letters, and the word “eleven” has eleven letters.

## # 6

There is such a parascience (pseudoscience) as numerology. This is a quasi-science about numbers. This pseudoscience studies their influence on the future and character of a person. Numerology has a long history and is similar to astrology.

## # 7

Zero is the only number that cannot be written in Roman numerals. Although in ancient times people knew about the existence of zero, they did not consider it a number at all. For example, the ancient Greek philosopher Aristotle did not consider zero to be a number, since you cannot divide by zero. Instead of a Roman numeral, the Latin word “nulla” was used to denote zero.

## No. 8

In our country “13” is considered unlucky. In Asia, “4” is considered an unlucky number. Even many buildings built in China and a number of other Asian countries do not have a 4th floor (the 3rd is immediately followed by the 5th).

In Asia, even a phobia such as tetraphobia has emerged. This is an irrational fear of the “four”.

We have developed such a phobia as triskaidekaphobia – this is a superstitious fear of the “thirteen”.

## No. 9

You’ve heard of magic 1089. Why is it magic? Example. Pick any number with three different digits, for example 254. Write it down in reverse order 452 and subtract 254 from it. You get 198. Now add 198 written backwards. You get the following: 198 + 891 = 1089.

Let’s take another three-digit number, for example, 762. Write it backwards and subtract it. 267-762 = 495. Now we add 495 and 594, we get the same 1089.

## No. 10

Do you know what will happen after a million? After a million comes billion, after billion trillion, after trillion quadrillion, after quadrillion quintillion, then sextillion, then septillion, then octillion, nonillion, decillion and undecillion.

## No. 11

Roman numerals were invented as a means of trading. The record-keeping form was used by the Romans as a means of easily valuing various goods and services, and was widely used throughout the Roman Empire for day-to-day processes. After the Roman Empire fell, Roman numerals were still used throughout Europe. Their use in Europe ceased around the 1600s.

## No. 12

Six is the smallest perfect number. In number theory, perfect is a positive integer equal to the sum of its positive divisors. For example “6” it looks like this: 1 + 2 + 3 = 6, where 1,2 and 3 are divisors of six. Following “6” is “28”. It consists of the sum of its 5 divisors – 1 + 2 + 4 + 7 + 14.

## No. 13

The number system (decimal system) that we use today is in fact based on the Hindu-Arabic system. This system was developed more than 1000 years ago, but it began to be used in Europe only in the 15-16th century, thus replacing the Roman numeral system.

## No. 14

Many people have their favorite and “lucky” numbers. The most popular is “7”. According to research, approximately 10% (9.7% to be more precise) of people chose the “seven”.

## No. 15

On the Internet, to express laughter, they often write “hahaha”. But did you know that Thais often write “555” during correspondence to express laughter? What is causing this? The fact is that 5 in Thai is written as ห้า (“ha” in Russian transcription). That is, if we write 55, then in Thai it will sound like “haha”, if 555 – “hahaha”.

## No. 16

Pythagoras of Samos, an ancient Greek philosopher and mathematician who lived in the 6th century BC, believed that odd numbers were male and even numbers were female. Pythagoras also believed that “3” is the “fruit of marriage” (the sum of 2 (even) and 1 (odd)), so he said that the “three” is perfect.

## No. 17

There is such an ailment as dyscalculia. This is a syndrome in which a person is unable to learn arithmetic. People with dyscalculia cannot recognize the number of objects in their field of vision. For example, if there are 5 books on the table, then a person cannot understand that there are five of them. Also, people with dyscalculia cannot understand why 2 + 2 = 4 and 5-3 = 2.

## No. 18

The largest number so far is the Graham number, named after Ronald Graham. It’s even bigger than googol and googolplex. In 1980, it got into the Guinness Book of Records.

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