Prime Numbers and Cryptography

Algorithms for finding prime numbers date back at least as far as ancient Greece, where mathematicians used a straightforward approach known as the Sieve of Erastothenes. The Sieve of Erastothenes works as follows: To find all the primes less than n, begin by writing down all the numbers from 1 to n in sequence. Then cross out all the numbers that are multiples of 2, besides itself (4, 6, 8, 10, 12, and so on). Take the next smallest number that hasn’t been crossed out (in this case, 3), an...

 The Gevulot

‘It’s a nice thought.’ He offers her his hand. ‘I am Paul. I got a little lost: all those moving streets. I was hoping you could tell me the way out.’ A trickle of emotion bleeds through his rough visitor’s gevulot: a sense of unease, a weight, a guilt. Xuexue can imagine the old man of the sea sitting on his back. It feels very familiar. And suddenly it is more important to talk to the stranger than to smile at the robot. ‘Sure I can,’ she says. ‘But why don’t you stay ...