# 13

13 is a prime number.

1, 1, 2, 3, 5, 8, 13...

A Fibonacci number.

A baker's, devil's or long dozen.

13 is the 'unlucky number' and Friday the 13th is supposed to be particularly unlucky.

** If April 13 is a Friday,
how many days will it be to the next Friday the 13th? What is the shortest
possible interval between two Friday the 13ths? What is the longest possible
interval?**

* Triskaidekaphobia* is the fear of the number 13, for example, of 13 people sitting at a table.

'Eleven plus two' is an anagram of 'twelve plus one'.

Thirteen is the number of hearts in a pack of cards.

Rugby League is played with teams of 13 players.

Under British law, when you reach the age of 13 you can get a part-time job, but you cannot work for more than two hours on a school day or a Sunday.

** When you buy potatoes in
a farm shop, they weigh them with an old pair of scales with weights.
Although they only have three weights, they can accurately weigh any whole
number of kilograms from 1 kg to 13 kg.**

** What three weights do they
have?**

If you look at this picture of a pine cone you can see a pattern of spirals coming out from the centre. They go in two directions. There are 13 spirals going anticlockwise and 8 going clockwise. Both 8 and 13 are Fibonacci numbers. If you count similar spiral patterns in other plants such as sunflowers you can count much larger Fibonacci numbers.

Quite ordinary pine cones show this pattern of 8 and 13 spirals, although it is sometimes easier to see it on the sides of the cone, instead of on the base as in the photograph here.

*In this
sign the sun has 13 rays. It is rare for artists to use 13-fold symmetry,
perhaps because their clients may be superstitious.*