From: David Hobby <[EMAIL PROTECTED]>
There are two kinds of divisibility tests. They aren't
usually given names, but let's call them "ending tests" and
"sum of digits tests". Working base 10, there are ending
tests for 2,4,8,... and 5,25,... as well as for their products.
(Let's ignore combined tests for products such as 6, since those
can always be created.)
In base 10, there are nice sum of digits tests for 3 and 9,
and a poor one for 11. (There's a really messy one for divisibility
by 7 as well, illustrating that it is always possible to produce
a poor test.) The tests for 3 and 9 are based on the fact that
10 = 9 + 1, and the test for 11 uses that 100 = 9*11 + 1.
What are the divisibility tests for 7 and 11?
-bryon
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