Casting out nines ? http://mathforum.org/library/drmath/view/72752.html "check digit" ? http://en.wikipedia.org/wiki/Check_digit www.math.unl.edu/~mbrittenham2/classwk/203f07/quiz/203words1.pdf "http://math.boisestate.edu/~tconklin/MATH124/Main/Readings/3 Number Theory/PDFs/Number Theory/Modular Arithmetic in the Marketplace.pdf" At 10:44 AM 2/25/2010, Dan Asimov wrote:

Eric wrote:

<< Does the operation going from 178 to 7 have a name (other than "modulus 9 digitsum")?

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Besides "digital root", it's also known as the remainder mod 9.

--Dan