43. DIVISIBILITY TESTS | |||
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Pick-up the technique

Add the digits; Repeat till get single digit; Test by 3.

Subtract

Repeat until you get a small enough number; Test by 7.

Sum of odd-place digits & sum of even-place digits differ by multiple of 11 (0, 11, 22,

Add

Repeat if needed; Test by 13.

We shall denote

We shall denote

More divisibility tests, using this notation,