# Try this: The sieve of Eratosthenes

You will need

* A grid containing each of the numbers from one to one hundred. You can download one here
* Several colours of pencil or texta

What to do

In this activity, you will find all the prime numbers under 100. A prime number can’t be divided evenly by any whole number except one and itself. For example, three is a prime number. Six can be divided evenly by two and three, so it isn’t a prime.

1. Start out by circling 2. Two is a prime number because it can only be divided by one and two. However, any other number that is divisible by two isn’t prime. Using one colour of pencil or texta, cross out every other even number. You should find they form neat rows.
2. Now circle 3. Three is also a prime number, but any other multiple of three is not, so cross out all the multiples of three with a different coloured texta. You’ll notice these run in diagonal lines across the page. Continue reading Try this: The sieve of Eratosthenes

# Searching for the smallest, hardest Sudoku

This Sudoku has 30 clues, but enthusiasts have found examples with only 17.

Sudoku are popular puzzles that can be seen in newspapers and puzzle books all around the world. The aim is to have a number in each one of the 81 boxes that make up the puzzle, while following certain rules. Some of the boxes start out with numbers in them already to act as clues. These clues make sure there is only one solution to the puzzle.

Most Sudoku have around 25 clues, but enthusiasts have long been interested in how few clues a Sudoku could have and still lead to only one answer. There are several Sudoku with only 17 clues, but no one had ever found a 16 clue Sudoku, so Gary McGuire from University College Dublin decided to look for one.

His strategy was simple – write down every possible answer, and then check to see if any of them could be posed as a 16 clue puzzle. Although the strategy was simple, it wasn’t going to be easy – there are 6 670 903 752 021 072 936 960 different possible answer grids to be checked. Continue reading Searching for the smallest, hardest Sudoku