Every single mathematician of numerologist will tell you that the number nine (9) is special, and every single esoteric person will tell you that the number 9 (nine) is also magical, but why is that? Before we dive into the magic of this number, let us be amused by these fine five examples:

•  In this article the symbol ">>" means "which is"

1) Any number added to 9 equals to the sum of those numbers

• n + 9 = ∑n ; it is like number 9 is invisible and does not count, for example 52 + 9 = 61 >> 6 + 1 =7 as 5 + 2 = 7
• 1971 >> 1 + 9 + 7 + 1 = 18 >> 1 + 8 = 9 where 1 + 7 + 1 = 9

2) Any number subtracted by 9 also equals the sum of those numbers

• n - 9 = ∑n ;because rule number 1 is true, this rule is bound to be true too! 52 - 9 = 43 >> 4 + 3 = 7 as 5 + 2 = 7
• A stairs has the same number of steps whether you climb up or down

 The sum of any number where 9

 https://www.youtube.com/watch?v=ab_dY3dZFHM

References

https://www.scientificamerican.com/article/the-magic-of-number-9-part-1/

http://burningmath.blogspot.com/2013/12/the-magic-number-nine.html

https://en.wikipedia.org/wiki/Cynthia_Longfield