Re: [math-fun] What is the name of [mod 9 digitsum]
Christoph wrote: << Dan wrote: << Besides "digital root", it's also known as the remainder mod 9.
Looking at the OEIS sequence that Neil mailed, I just recognized it is not as simple, since 0 is not a regular digital root (except for 0 itself).
For non-zero multiples of 9 the digital root is 9.
You are right, of course. For positive numbers that are multiples of 9 it is the remainder mod 9 (i.e., 0) plus 9. << I remember that Martin Gardner has some nice magic tricks based on the digital root in his 1956 book "Mathematics, Magic and Mystery" (Dover), ("Mathematik und Magie", Dumont in German). At least as boy I enjoyed them. :)
Yes, this is the first Martin Gardner book I found (at age 8) and I loved it. --Dan ________________________________________________________________________________________ "Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read." --Groucho Marx
For non-zero multiples of 9 the digital root is 9.
You are right, of course. For positive numbers that are multiples of 9 it is the remainder mod 9 (i.e., 0) plus 9.
I was in twelfth grade in a Toronto high school in 1968 when I contributed an article called "digital roots" to a student-created, mimeographed, one-off publication called Samhita. Because it seemed sensible to equate the digital-root concept to a remainder-when- divided-by-nine, I suggested that "no distinction is made between 0 and 9". :) http://chesswanks.com/pot/Samhita/pages/page_7.html
participants (2)
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Dan Asimov -
Hans Havermann