**ajh@sdcsvax.UUCP (Alan Hu)** (08/29/83)

Back in my old Junior High Math Team Days, I had rules for all the numbers 1-16. (You had to do those problems fast!) Since someone already gave the rule for 7, I think I'll give the rule for 13. Note: I am not a number theoretician, and I make no claims as such. I don't even know if I can prove this. I was given this rule, and I accepted it on faith. It seems to work. Divisibility by 13: A number n is divisible by 13 iff the number floor(n/10) + 4 * (n mod 10) is divisible by 13. For example, take 160485 16048 + 4 * 5 = 16068 1606 + 4 * 8 = 1638 163 + 4 * 8 = 195 19 + 4 * 5 = 39 (If you can't tell by now, you're hopeless!) 3 + 4 * 9 = 39 (And you deserve to sit here and look at 39's . . . forever. By the way, can any of you derive a function to show which numbers won't reduce by this method? You know the number must be small, because 4 times the units digit must be greater than 9 times the rest of the number.) --Alan J. Hu sdcsvax!ajh