In set notation this is B = {1, 4, 9}. 30 people answered a survey about whether they have a dog or a cat. The Venn diagram shows the information from the survey. 13 people have got a dog.

You may well be familiar with Venn diagrams – the ubiquitous pictures of typically two or three intersecting circles, illustrating the relationships between two or three collections of things.

A new rose has blossomed in the garden of mathematics: a flowery Venn diagram for 11 sets of objects. Venn diagrams use overlapping circles to show all possible relationships between sets.