4 ms·

I've heard 91 called the "smallest non-obvious composite". Assume that testing for divisiblity by 2, 3, 5, and 11 is easy - for 2 and 5 you just look at the las

I've heard 91 called the "smallest non-obvious composite". Assume that testing for divisiblity by 2, 3, 5, and 11 is easy - for 2 and 5 you just look at the last digit, for 3 you take the sum of the digits, and with two-digit numbers a number is divisible by 11 if and only if both its digits are the same. Also assume that you can recognize squares. Then 91 = 7*13 is the first number that you won't recognize as composite using these test.

sdenton4 11y ago
The more general test for eleven is to check that the alternating sum of the digits is zero.
- cousin_it 11y ago
  Counterexample: 209. The even more general test is that the alternating sum of digits is divisible by 11.
  - sdenton4 11y ago
    (I'll pretend to defend myself and say I meant zero mod eleven. :) )