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xordan
guest

Jun2707, 08:35 PM (EST) 

"digits sum"

Hello: I have found that if two numbers A and B sum C (A+B=C), the sum of the digits of A + the sum of the digits of B is similar to the sum of the digits of C. (reduced to a digit) Does this property have a demonstrable foundation as theorem? It is alone a curiosity that has arisen when I play with numbers in search of finding a easy test of primality. Investigating this property I found your site that I find very interesting and informative and it contains similar topics those that I ask. Thank you for your time and attention. xordan.tom@gmail.com P.S.: Original in spanish 

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alexb
Charter Member
2039 posts 
Jul0407, 01:37 AM (EST) 

1. "RE: digits sum"
In response to message #0

>Hello: >I have found that if two numbers A and B sum C (A+B=C), the >sum of the digits of A + the sum of the digits of B is >similar to the sum of the digits of C. (reduced to a digit) The sum of the digits (reduced to a single digit) is known as digital root. This has the property, indeed, that the digital root of the sum equals to the (reduced if necessary) sum of digital roots. The same holds for products. The digital root of a number divisible by 9 is 9. Otherwise, the digital root of a number N is N (mod 9). 

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