### Special properties of number 30 (Today’s Date)

1. 30 is the sum of the first four squares, which makes it a square pyramidal number.
2. The icosahedron and the dodecahedron are Platonic solids with 30 edges.
3. 30 is a Harshad number ( an integer that is divisible by the sum of its digits)
4. The atomic number of zinc is 30
5. The minimum age for United States senators.
6. February 30 is usually used as a sarcastic date for referring to something that will never happen or will never be done.

### Special properties of number 29 (Today’s Date)

1. None of the first 29 natural numbers have more than two different prime factors. This is the longest such consecutive sequence.
2. 29 is the sum of three consecutive squares i.e. 29 =  2^2 + 3^2 + 4^2
3. The number of days February has in leap years.
4. The atomic number of copper
5. Saturn requires over 29 years to orbit the Sun.
6. Number of states in india

### Special properties of number 28 (Today’s Date)

1. 28 is a triangular number since it is the sum of the first seven counting numbers: 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28.
2. 28 is also a perfect number since the sum of its aliquot divisors (factors that are less than the number itself) is also 28. The aliquot factors of 28 are 1, 2, 4, 7, and 14, and 1 + 2 + 4 + 7 + 14 = 28. The next smallest perfect number is 6, and the next highest perfect number is 496.
3. 28 is an example of a happy number. When you square the digits, add them, and repeat this process on the resulting number, you eventually end up with 1:
2^2 + 8^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1
4. The sum of the first five prime numbers is 2 + 3 + 5 + 7 + 11 = 28.
5. 28 is a hexagonal number, but not a centered hexagonal number. Every hexagonal number is also a triangular number, but not every triangular number is a hexagonal number.
6. A polygon with 28 sides is called an icosikaioctagon.
7. Between the years 2000 and 2099, the calendar repeats every 28 years. It also cycles every 28 years between 2101 and 2199.

### Special properties of number 27 (Today’s Date)

1. Twenty-seven is a perfect cube = 3^3.
2. In base 10, it is the first composite number not evenly divisible by any of its digits.
3. 27 is the only positive integer that is 3 times the sum of its digits.
4. 27 is a Harshad number (a number divisible by sum of its digits).
5. 27 is the smallest 2-digit number in which the sum of digits is equal to the sum of prime factors, e.g.: 27 = 3 x 3 x 3 and 2 + 7 = 3 + 3 + 3 = 9
6. 23 is the largest integer which is the sum of the digits of its cube:
27^3 = 19,683 and 27 = 1 + 9 + 6 + 8 + 3
7. 27 is the number of "cubies" in a Rubik's cube.
8. The numbers of atoms in the body of an average male (about 80 kg) is about 10^27.
9. The Chemical Element Cobalt has an atomic number of 27.
10. The primitive Greek alphabet had 27 letters.
11. There are 27 signs of the zodiac in Indian astrology

### Special properties of number 26 (Today’s Date)

1. 26 is the only number sandwiched between a square (5^2) and a cube (3^3).
2. There are 26 letters in the English alphabet.
3. In base ten, 26 is the smallest number that is not a palindrome to have a square (26^2 = 676) that is a palindrome.
4. In a normal deck of cards, there are 26 red cards and 26 black cards.
5. The Chemical Element Iron has an atomic number of 26.
6. 26 is the number of bones in the normal human Foot and Ankle.
7. There are 26 weeks in half a year.

### Special properties of number 25 (Today’s Date)

1. It is a square number, being 5^2 = 5 × 5. It is the smallest square that is also a sum of two (non-zero) squares: 2^5 = 3^2 + 4^2.
2. 25 is the sum of the first 5 odd numbers(= 1 + 3 + 5 + 7 + 9).
3. Couples who have been married for 25 years celebrate their Silver Wedding.
4. The Chemical Element Manganese has an atomic number of 25.
5. 25 is the number of days approximately that takes the sun to do a complete rotation on itself.

### Special properties of number 24 (Today’s Date)

1. 24 is the factorial of 4 (24 = 4!).
2. 24 is an Abundant Number. In number theory, an abundant number or excessive number is a number for which the sum of its proper divisors is greater than the number itself. The first few abundant numbers are 12, 18, 20, 24, 30, 36, ... There are only 21 abundant numbers less than 100, and they are all even.
3. 24 is the largest number divisible by all numbers less than its square root.
4. It is a highly composite number, having more divisors than any smaller number.
5. 24 is a semiperfect number, since adding up all the proper divisors of 24 except 4 and 8 gives 24.
6. Subtracting 1 from any of its divisors (except 1 and 2, but including itself) yields a prime number; 24 is the largest number with this property.
7. 24 is a Harshad number (number divisible by its sum of digits).
8. The product of any four consecutive numbers is divisible by 24.
9. 24 is the only nontrivial solution to the cannonball problem, that is: 1^2+2^2+3^2+...+24^2 is a perfect square (=70^2). (The trivial case is just 1^2 = 1^2.)
10. 24 is the number of Tirthankaras (in Janism)
11. 24 is the number of spokes in the Ashok Chakra
12. 24 carats represents 100% pure gold.
13. There are 24 hours in a day.
14. 24 is the number of letters in both the modern and classical Greek alphabet.
15. The Chemical Element Chromium has an atomic number of 24.

### Special properties of number 23 (Today’s Date)

1. According to the birthday paradox, in a group of 23 (or more) randomly chosen people, the probability is more than 50% that some pair of them will have the same birthday. For 60 or more people, the probability is greater than 99 per cent!
2. 23 is the smallest prime number with consecutive digits.
3. 23! is 23 digits long.
4. Twenty-three is the sum of three other, consecutive, prime numbers; 5, 7 and 11. It is the first prime number showing this characteristic.
5. The sum of the first 23 primes is 874, which is divisible by 23, a property shared by few other numbers.
6. Repeat the digit 1, 23 times like this: 11,111,111,111,111,111,111,111 and you obtain a prime number.
7. Every positive whole number can be written as the sum of eight cubes (including 0^3 when necessary) except 23 (and 239). Those two numbers require 9 cubes.
8. Nobel Prize-winning economist John Forbes Nash, the inspiration for the film A Beautiful Mind, was obsessed with the number 23 and it featured prominently in his nervous breakdown. He claimed that Pope John XXIII was in fact himself, the evidence being that 23 was his favorite number. Nash also published only 23 scientific articles.
9. The axis of the planet Earth is 23.5 degrees to the vertical. The tropics of Cancer
and Capricorn are at 23.5˚ north and south respectively.
10. The Chemical Element Vanadium has an atomic number of 23.
11. Normal human sex cells have 23 chromosomes. Each parent contributes 23 chromosomes to the start of human life. The nuclei of cells in human bodies have 46 chromosomes made out of 23 pairs.

### Special properties of number 22 (Today’s Date)

1. Twenty-two is a pentagonal number.
2. 22 is a palindromic number in decimal system
3. 22 divided by 7 approximates the irrational number π, the ratio of the circumference of a circle to its diameter.
4. There are, in fact, precisely 22 ways to express 8 as a sum of positive integers. i.e. 8 can be partitioned in 22 ways:
8, 7+1, 6+2, 6+1+1, 5+3, 5+2+1, 5+1+1+1, 4+4, 4+3+1, 4+2+2, 4+2+1+1, 4+1+1+1+1, 3+3+2, 3+3+1+1, 3+2+2+1, 3+2+1+1+1, 3+1+1+1+1+1, 2+2+2+2, 2+2+2+1+1, 2+2+1+1+1+1, 2+1+1+1+1+1+1, and 1+1+1+1+1+1+1+1
5. 22 is the smallest number which can be expressed as the sum of 2 primes in 3 ways: 3 + 19, 5 + 17, and 11 + 11
6. 22 is the number of different ways of linking 5 hexagons together.
7. The Chemical Element Titanium has an atomic number of 22.
8. The Human head is constituted of 22 bones: 8 for the cranium and 14 for the face.
9. By the old English measuring system there are 22 yards in a chain. This is the
length of a cricket pitch.
10. There are 22 players on the field in American Football, soccer and field hockey.

### Special properties of number 21 (Today’s Date)

1. Twenty-one is a Fibonacci number, a Harshad number, triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.
2. 21 is the sum of the first six natural numbers (1+2+3+4+5+6=21), making it a triangular number. Also, the number of spots on a standard cubical (six-sided) die (1+2+3+4+5+6)
3. 21 is the smallest non-trivial example of a Fibonacci number whose digits are Fibonacci numbers and whose digit sum is also a Fibonacci number.
4. 21 is a repdigit in base 4 (1114).
5. 21 is the third star number. Star numbers can be represented by a square with a triangle on each side.
6. 221 - 21 is prime.
7. The current century—spanning the years from 2001 to 2100—is referred to as the 21st century.
8. 21 grams is the weight of the soul, according to research by Duncan MacDougall, generally regarded as meaningless.
9. The Chemical Element Scandium has an atomic number of 21.
10. There are 21 letters in the Italian alphabet.

### Special properties of number 20 (Today’s Date)

1. 20 = 2 + 4 + 6 + 8
2. 20 = 1 + 3 + 6 + 10. i.e. 1 + (1 + 2) + (1 + 2 + 3 ) + (1 + 2 + 3 + 4 )
3. 20 is an Abundant Number. In number theory, an abundant number or excessive number is a number for which the sum of its proper divisors is greater than the number itself. The first few abundant numbers are 12, 18, 20, 24, 30, 36, ... There are only 21 abundant numbers less than 100, and they are all even.
4. 20 is a Tetrahedral Number.
5. One month in the religious Mayan calendar contained twenty days.
6. 20 is the base of the ancient Mayan numeral system.
7. A set of 20 units may also be referred to as a score.
8. The Chemical Element Calcium has an atomic number of 20.

### Special properties of number 19 (Today’s Date)

1. 19 is a prime number.
2. 19 is the maximum number of 4th powers needed to sum to any number
3. 19 is the largest prime number that is palindromic in Roman numerals (XIX).
4. 19 is an Octahedral Number.
5. The Chemical Element Potassium has an atomic number of 19.
6. "The Sun" is numbered with 19 in Tarot cards.

### Special properties of number 18 (Today’s Date)

1. 18 is a Heptagonal Number.
2. 18 is an Abundant Number. In number theory, an abundant number or excessive number is a number for which the sum of its proper divisors is greater than the number itself. The first few abundant numbers are 12, 18, 20, 24, 30, 36, ... There are only 21 abundant numbers less than 100, and they are all even.
3. 18 is a Lucas Number (No. from the sequence 1, 3, 4, 7, 11, 18, 29, 47, 76, 123, … where first two numbers are fixed as 1 and 3, next number is the sum of its two preceding terms).
4. The 18 Regular Solids: the 5 Platonic Solid + the 13 Archimedean Solids.
5. 18 is the area and also the perimeter of a rectangle with 6 and 3-unit sides.
6. 18 is the only number that is twice the sum of its digits.
7. On Earth, the length of a day 900 million years ago was only about 18 hours.
8. There are 18 chapters in the Hindu epic tale Bhagavad Gita.
9. The Chemical Element Argon has an atomic number of 18.

### Special properties of number 17 (Today’s Date)

1. 17 is a prime number.
2. 17 is the sum of the first 4 prime numbers: 17 = 2 + 3 + 5 + 7
3. 17 is a palindromic number in binary system (10001).
4. 17 is a Leyland number, 17 = 3^2 + 2^3(In number theory, a Leyland number is a number of the form x^y + y^x where x and y are integers greater than 1. They are named after the mathematician Paul Leyland. The first few Leyland numbers are
8, 17, 32, 54, 57, 100, 145, 177, 320, 368, 512, 593, 945, 1124…
5. 17 is the 7th prime number and the only prime of the form p^q + q^p, where p and q are prime: 17 = 2^3 + 3^2
6. 17 is the lowest possible number of givens for a Sudoku puzzle with a unique solution (this was proven in 2012 by Gary McGuire, Bastian Tugemann, and Gilles Civario).
7. No odd Fibonacci Number is divisible by 17.
8. abcdefghabcdefgh is divisible by 17 (replace each letter with a digit. E.g. 1234567812345678). The reason is that any number of that form is a multiple of 100,000,001 - which is divisible by 17.
9. A regular 17-gon can be constructed by using rule and compasses only.
10. There are exactly 17 ways to express 17 as the sum of 1 or more primes i.e. 17 is the only integer which is equal to the number of prime partitions of itself.

11. The famous problem of the 17 camels: a sheik has 3 children and owns 17 camels. His will stipulates that the eldest is to receive half his property; the second son is to receive the third of his property; and the third one, the ninth of his property. On his death, how would the sons share out the inheritance? Solution: They borrow a camel, share out, and give back the camel: (17+1)/2 + (17+1)/3 + (17+1)/9 = 17 camels.

12. The Chemical Element Chlorine has an atomic number of 17.
13. There are 17 muscles in the Human tongue.
14. A day on Uranus lasts for 17 hours.

### Special properties of number 16 (Today’s Date)

1. 16 is the only number of the form x^y = y^x with x and y being different integers.
2. 16 is a Square Number (4x4).
3. 16 is the maximal number of regions into which 5 lines divide a plane.
4. 16= 1 + 3 + 5 + 7 i.e. sum of first four odd numbers
5. The base 16 notational system for representing real numbers, called hexadecimal, is particularly important and used extensively in computer science, since four bits (each consisting of a 'one' or 'zero') can be succinctly expressed with just a single hexadecimal digit. The 16 distinct digits used to represent numbers in hexadecimal notation are: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, and F.
6. The Chemical Element Sulfur has an atomic number of 16.

### Special properties of number 15 (Today’s Date)

1. 15 is a triangular number  as 15 = 1 + 2 + 3 + 4 + 5
2. 15 is a hexagonal number
3. 15 is a palindromic number in binary system (1111)
4. 15 is the magic constant for a 3×3 magic square, the smallest possible nontrivial magic square. An N×N magic square consists of the N2 integers from 1 to N arranged in an N×N square grid, such that the sum of any row, or any column, or of one of the two diagonals, is equal to any other. There is only one solution for a 3×3 square, not counting its rotations and reflections:
4  9  2
3  5  7
8  1  6
This magic square was known to the Chinese at least 3000 years ago and is called the Lo Shu.
5. The Chemical Element Phosphorus has an atomic number of 15.
6. The only 15-letter word that can be spelled without repeating a letter is "uncopyrightable".

### Special properties of number 14 (Today’s Date)

1. 14 is a square pyramidal number as 14 = 1 + 4 + 9 = 12 + 22 + 32
2. In September 1752, Great Britain switched from the Julian Calendar to the Gregorian Calendar. In order to achieve the change, 11 days were 'omitted' from the calendar: the day after 2 September 1752 was 14 September 1752.
3. The Chemical Element Silicon has an atomic number of. 14.

### Special properties of number 13 (Today’s Date)

1. 13 is a Fibonacci number
2. 13 is the number of Archimedian solids.
3. 13 is a Prime Number. 13 is a Fibonacci Number.
4. The Chemical Element Aluminum has an atomic number of 13.
5. The card deck includes 13 hearts, 13 spades, 13 squares, 13 clubs.
While the Earth revolves once, the Moon revolves 13 times.
6. There is always at least one Friday 13th in each year.
The probability of being born on a Friday the 13th is about 1/214
7. According to historians, there were 13 people at Christ's Last Supper and Christ was crucified on Friday 13th. So, Friday 13th is considered as unlucky.
8. The fear of the number 13 is called 'Triskaidekaphobia'.

### Special properties of number 12 (Today’s Date)

1. 12 is a pentagonal number
2. 12 is the smallest abundant number.
3. 12 is a pentagonal number.
4. 12 is the smallest number with exactly six divisors.
5. 12 is the largest known even number expressible as the sum of two primes in one way (5 + 7).
6. A dozen is a group or set of 12.
7. The Cube is a Platonic Solid with 12 Edges.
8. The Dodecahedron is a Platonic Solid with 12 Faces.
9. The Icosahedron is a Platonic Solid with 12 Vertices.
10. The Chemical Element Magnesium has an atomic number of 12.
11. The 12 Star signs: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagitarius, Capricorn, Aquarius and Pisces.

### Special properties of number 11 (Today’s Date)

1. 11 is a palindromic number in decimal system. It is only palindromic prime with an even number of digits.
2. 11 is a Lucas Number.
3. 6 can be Partitioned in 11 ways.
4. 11 is the third honest number, because 11 = "two plus nine".
5. An average room holds 11 x 100 pounds of air!
6. A million seconds is 11 days and a half.
7. The Chemical Element Sodium has an atomic number of 11.
8. The sun spot cycle repeats about every 11 years.

### Special properties of number 10 (Today’s Date)

1. 10 is the base of our number system.
2.10 is the sum of the first 3 prime numbers: 10 = 2 + 3 + 5
3. 10 is both a triangular (1 + 2 + 3 + 4 = 10) and tetrahedral number.
4.10 is asemiprime number. A semiprime is a natural number that is the product of two (not necessarily distinct) prime numbers. The semiprimes less than 100 are 4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, 65, 69, 74, 77, 82, 85, 86, 87, 91, 93, 94, and 95.
The Chemical Element Neon has an atomic number of 10.
5. 10! = 6! x 7! = 3! x 5! x 7! (Unique solution to the factorial equation n! =a! x b! x c! with consecutive prime factors)
6. In most advertisements, including newspapers, the time displayed on a watch is approximately 10:10. The reason for this is to "frame" the logo of the watch maker. The hands at that number cause the eyes to look right at the logo!
7. Marion's theorem states that the area of the central hexagonal region determined by trisection of each side of any triangle ABC and connecting the corresponding points with the opposite vertex is exactly one tenth (1/10) the area of the whole triangle ABC.

### Special properties of number 9 (Today’s Date)

1. 9 is the largest single-digit number.
2. 9 is a Square Number (3x3).
3. 9 is a palindromic number in decimal system
4. 9 is a palindromic number in binary system (1001)
5. The Chemical Element Beryllium has an atomic number of 9.
6. Water expands by about 9% as it freezes.
7. A full moon is nine times brighter than a half moon.

### Special properties of number 8 (Today’s Date)

1. The smallest "non-trivial" cube: 8=23=2×2×2.
2. 8 is a Fibonacci number. Also it is the largest cube in the Fibonacci sequence.
3. 8 is an Octagonal Number.
4. 8 is the smallest sum of two factorials of distinct primes: 2! + 3!
5. When specifying directions on a map, most people choose from one of these 8 directions: north, northeast, east, southeast, south, southwest, west, and northwest.
6. In three-dimensional space there are 8 "diagonal" ways to move, corresponding to the eight "octants" you get if you divide the three-dimensional space with three mutually-perpendicular planes.
7. The Chemical Element Oxygen has an atomic number of 8.

### Special properties of number 7 (Today’s Date)

1. 7 is a Prime Number.
2. 7 is a palindromic number in decimal system. Also 7 is a palindromic number in binary system (111).
3. 7 is a Mersenne Number (number of the form 2^n -1)
4. 7 is an Octahedral Number.
5. 7 is a Lucas Number.
6. Sum of the first 4 Fibonacci numbers= 1 + 1 + 2 + 3 = 7
7. 7 is the smallest number of sides of a regular polygon that is not constructible by straightedge and compass.
8. Topologists have been able to prove that 7 colors may be needed on a donut shaped map to ensure that no adjacent areas are the same.
9. The smallest positive integer whose reciprocal has a pattern of more than one repeating digit: 1/7 = 0.142857142857...
10. The opposite sides of a die cube always add up to 7.
11. A square piece of paper cannot be folded in half more than 7 times... However, Britney Gallivan (of Pomona, California) found a formula that tells us to successfully fold paper 12 times, we would need about 1.2 km of paper... and she proved it!
12. The 7 colors of the rainbow.
13. There are 7 days in creation, 7 days in the week, 7 phases of the moon etc
14. The 7 wonders of the ancient world are:
- Great Pyramid of Giza,
- Hanging Gardens of Babylon,
- Temple of Artemis at Ephesus,
- Statue of Zeus at Olympia,
- Mausoleum of Maussollos at Halicarnassus,
- Colossus of Rhodes,
- Lighthouse of Alexandria.
15. Adolf Hitler was reputed to take his coffee or tea with seven teaspoons of sugar.
16. Nitrogen is a chemical element with atomic number 7.

### Special properties of number 6 (Today’s Date)

1. 6 is the smallest perfect number, that is a number whose divisors add up to itself, e.g.: 1 x 2 x 3 = 1 + 2 + 3 = 6
2. 6 (= 1 + 2 + 3) is a triangular number
3. 3! = 6
4. 6 is a Hexagonal Number.
5. 6 is a congruent number because it is the area of a 3, 4, 5 triangle (a congruent number is an integer that is the area of a right triangle with three rational number sides).
6. 6 is the smallest number of colors needed to color the regions on a Möbius strip. A Möbius strip is a continuous closed surface with only one side; formed from a rectangular strip by rotating one end 180 degrees and joining it with the other end.
7. 6 circles of the same size (try this with 6 coins of the same denomination) will always perfectly surround, all touching, without gaps, 1 circle of that same size.
8. The Chemical Element Carbon has an atomic number of 6.

### Special properties of number associated with Today’s Date

Dear All,
I have been collecting information from various resources regarding the special properties being associated with today's date.
It's already 5th June today. So, today I will share properties from the dates (numbers) 1 to 5.
In future I would prefer to post day wise.
In case you also wish to share something interesting kindly do it.
Any suggestions are also welcome.
Amit Bajaj

Special properties of number 5 (Today’s Date)

### 5. 5is a Pentagonal Number.

6. The Chemical Element Boron has an atomic number of 5.
7. The five natural senses of sight, hearing, smell, taste, and touch.

8. The 5 Elements: Fire, air, water and Earth and Spirit.

Special properties of number 4 (Today’s Date)
1.  Four is the smallest composite number, its proper divisors being 1 and 2.
2. 4 is the smallest number of colors sufficient to color all planar maps.
3.  4 is a palindromic number in decimal system.
4. The word 'four' has 4 letters and is the smallest honest number. Honest numbers are numbers n that can be described using exactly n letters in standard mathematical English.
5. A 4 x 4 square has an area equal to its perimeter.
6. The atomic number of beryllium.
7. Using four 4's and any operations, try to write equations that have the integers from 1 to 100 as the answer (see example below):
1 = 44/44
2 = 4/4 + 4/4
3 = (4 + 4 + 4)/4
4 = 4(4 - 4) + 4, etc...
8. In Puruṣārtha, there are four aims of human life: Dharma, Artha, Kāma, Moksha.
9. The four stages of life Brahmacharya (student life), Grihastha (household life), Vanaprastha (retired life) and Sannyasa (renunciation).
10. Four seasons: spring, summer, autumn, winter
11. Four cardinal directions: north, south, east, west.
12. Four suits of playing cards: hearts, diamonds, clubs, spades.
13. There are four basic states of matter: solid, liquid, gas, and plasma.

Special properties of number 3 (Today’s Date)
1. 3 is a Fibonacci number
2. 3 is a triangular number3 is a palindromic number in decimal system
3. 3 is a palindromic number in binary system (11)
4. Three is the number of dimensions that humans can perceive. Humans perceive the universe to have three spatial dimensions

### 10. The triangle, a polygonwith three edgesand three vertices, is the most stable physical shape. For this reason it is widely utilized in construction, engineering and design.

Special properties of number 2 (Today’s Date)
1. 2 is the only even prime.
2. 2 is a Fibonacci number
3. 2 is a palindromic number in decimal system.
4. For any Polyhedron , V – E + F = 2 where V is the number of vertices, E is the number of edges, and F is the number of faces.
5. The atomic number of helium.
6. A honey bee must tap TWO million flowers to make ONE pound of honey!

Special properties of number 1 (Today’s Date)
1. 0! = 1
2. 1 is the atomic number of hydrogen
3.   Philo of Alexandria (20 BC – AD 50) regarded the number one as God's number, and the basis for all numbers
4. 1 is a Fibonacci number
5. 1 is a triangular number
6. 1 is a pentagonal number
7. 1 is a hexagonal number
8. 1 is a heptagonal number
9. 1 is a octogonal number
10. 1 is a palindromic number in decimal system
11. A list of primes to 10,006,721 published in 1914 by Derrick Lemher includes 1 as prime.
12. A, B , C , D , E, M, T, U, V , Y has one line of symmetry
13. Till 15th century 1 was not included in set of natural number. Simon Stevin included it in Natural number.