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