Re: [math-fun] What is the name of [mod 9 digitsum]
Eric wrote: << Does the operation going from 178 to 7 have a name (other than "modulus 9 digitsum")?
Besides "digital root", it's also known as the remainder mod 9. --Dan ________________________________________________________________________________________ "Outside of a dog, a book is man's best friend. Inside of a dog, it's too dark to read." --Groucho Marx
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. 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. :) Christoph
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Dan Asimov -
Pacher Christoph