Twenty one (21)
1. There are twenty one dots on a die. In a die numbers are marked from 1 to 6 and they add up to 21. 1 + 2 + 3 + 4 + 5 + 6 = 21
2. It is the product of two numbers 3 and 7 associated with good luck. 21 = 3 x 7
3. It is a triangular number.
1. There are twenty one dots on a die. In a die numbers are marked from 1 to 6 and they add up to 21. 1 + 2 + 3 + 4 + 5 + 6 = 21
2. It is the product of two numbers 3 and 7 associated with good luck. 21 = 3 x 7
3. It is a triangular number.
4. It is a Fibonacci number. The number in Fibonacci sequence is 1, 1, 2, 3, 5, 8, 13, 21, ---
5. It is the third star number. Star numbers can be represented by a square with a triangle on each side.
6. If a square number ends with xy xy xy xy xy then xy will be either of these- 21, 29, 61, 69 or 84.
508 853 989 x 508 853 989 = 258 932 38 21 21 21 21 21
7. It is the smallest Fibonacci number whose digits are Fibonacci number and whose sum of digit is also a Fibonacci number.
8. The atomic number of Scandium is 21.
9. The unique projective plane of order 4 has 21 points and 21 lines, with 5 points on each line and 5 lines through each point.
10. It is a Harshad Number.
11. 21 repeated twenty-one times, following 1, forms a smoothly undulating palindromic prime
121212121212121212121212121212121212121 is prime
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Dr. Rajesh Kumar Thakur
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