Remark for alternating sum: when the least significant digit is calculate as positive, the sum (when it is tranform into digit in such system) is a remainder of division N/D.
Find a number D, you're interested in its divisibility rule. When You don't see it, try General page.
It is a simple way to generate a rule using PU method. If You catch it, You can generate Your own rule for 47, 103, 989 and others...
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It is nice the PD method from this number.
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The metod PD finishes be nice. The method PU works nice with upgrade (use modulo all digits) .
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