26 Feb
2010
26 Feb
'10
8:09 p.m.
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