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21 is a Fibonacci Number
Leonard of Pisa described a series of numbers where each number is the sum of the two preceding numbers. Although he was not the first to conceive of such a series (which is described in ancient Sanskrit texts) the series is named after him. Leonard likened the series to a rabbit colony which is constantly propagating under certain mathematical rules. The number 21 is one of the numbers in the classic Fibonacci series (of note, that this series was published at the very beginning of the thirteenth century (1202) which again contains the number 21). The number 21 has other qualities as well…
21 is a Harshad Number
Harshad numbers are numbers which are divisible by the sum of their digits. All Harshad numbers are numbers which are divisible by the sum of their digits in any base used (there are only 4 of these). 21, which is the multiple of 7 and 3 is thus, a Harshad number (since 2 plus 1 equals 3). The word Harshad is derived from Sanskrit where it means ‘great joy’. This is perhaps part of what has served to make twenty one a lucky number in many games. In fact, the number 21 has special significance regarding Harshad numbers. This is because it has been proven that in base ten, the longest sequence of consecutive Harshad numbers possible (in base ten) is 20.
Is 21 Really Special?
In fact, the number 21 has more mathematical qualities as well. It is a Motzkin number (which is of great significance in geometry as well as other fields of mathematics) as well as a triangular number, an octagonal number and a composite number. In numerology 21 has particular significance due the fact that it is the product of 3 multiplied by 7 (both numbers of special significance in numerology). The number 21 also happens to be the sum of the digits from 1 to 5. How these qualities serve to influence the effects of the number 21 is beyond the scope of this article but it is always fun to speculate…
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