What would we do without referrers? I do not really know, but bloggers would have to do without a lot of funny search requests.

A few days ago, someon was looking for a Rechenregel wie man erkennt durch welche Zahlen eine Zahl teilbar ist. That roughly translates as rule how to recognize by which numbers a number is divisible. Anyway, I am happy to provide a few. All rules are equivalencies, that is the given number is divisible if the rule is fulfilled; otherwise, it is not divisible.

  1. every number is divisible by one 🙂
  2. the final digit is divisible by two
  3. the cross total (sum of digits) is divisible by three
  4. the number comprised of the final two digits is divisible by four
  5. the final digit is zero or five
  6. both rules 2 and 3 hold
  7. double the final digit and subtract it from the rest of the number; the result is divisible by seven; e.g. 203: 20-2*3=14 is divisible
  8. the number comprised of the final three digits is divisible by eight
  9. the cross total is divisible by nine
  10. the final digit is zero
  11. the alternating sum of digits is divisible by eleven, e.g. 135795: 1-3+5-7+9-5=0 is divisible
  12. rules 3 and 4 hold
  13. quadruple the final digit and add it to rest of the number; the result is divisible by thirteen; e.g. 403: 40+4*3=52; it is useful to apply the rule again: 5+4*2=13 isdivisible, so 52 is, and therefore 403 as well.

Well, that should suffice for now.

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