### 31.10.05

## 91

if you want to remember the prime numbers up to 100:

- 1 isn't a prime, unless you reckon it is, in which case it might be.
- the even numbers end in 0, 2 ,4 ,6 or 8 and aren't primes (except 2)
- multiples of three have digits which sum to a multiple of 3 and aren't primes (except 3)
- odd multiples of 5 end in 5 and aren't primes (except 5)
- multiples of 11 have two digits the same and aren't primes (except 11)
- 49 isn't a prime = 7*7. most people know that.
- 91 isn't a prime = 7*13. most people don't know that.

if you remember the primes up to 100 you can factorise (and test for primality) numbers up to 10,201 (= 101*101)