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From: ark@rabbit.UUCP
Newsgroups: net.math
Subject: divisibility by 7
Message-ID: <1842@rabbit.UUCP>
Date: Wed, 24-Aug-83 23:54:06 EDT
Article-I.D.: rabbit.1842
Posted: Wed Aug 24 23:54:06 1983
Date-Received: Fri, 26-Aug-83 04:29:11 EDT
Organization: Bell Labs, Murray Hill
Lines: 15

To tell if an integer n is divisible by 7, repeat the following:

	n := floor(n / 10) - 2 * (n mod 10)

You will evetually get a number that you can check at a glance.
For instance:

	142857

	14285 - 14 = 14271
	1427 - 2 = 1425
	142 - 10 = 132
	13 - 4 = 9

9 is not divisible by 7, so neither is 142857.