This is from Paul Payton. -- Rich ------------------------------------------ Date: Thu, 25 Feb 2010 10:43:05 -0700 Subject: RE: What is the name of [mod 9 digitsum] From: "Payton, Paul" <paul.payton@lmco.com> I would call it "additive persistence". http://en.wikipedia.org/wiki/Persistence_of_a_number -----Original Message----- From: math-fun-bounces@mailman.xmission.com [mailto:math-fun-bounces@mailman.xmission.com] On Behalf Of Eric Angelini Sent: Thursday, February 25, 2010 9:29 AM To: math-fun Subject: [math-fun] What is the name of [mod 9 digitsum] Hello Math-Fun, is there a name for this operation : - start with 178 for instance; - add the digits: 1+7+8 => 16 - add the digits again: 1+6 => 7 Does the operation going from 178 to 7 have a name (other than "modulus 9 digitsum")? Thank you, Best, É.