# Sequences and PatternsFibonacci Numbers

Imagine that you’ve received a pair of baby rabbits, one male and one female. They are very special rabbits, because they never die, and the female one gives birth to a new pair of rabbits exactly once every month (always another pair of male and female).

In the following month you would have 13 pairs of rabbits: the 8 ones from the previous month, plus 5 new sets of babies. Can you detect a pattern in this sequence?

The number of rabbits in a particular month is *previous two* terms in the sequence, to get the next one. The sequence starts with two 1s, and the

Can you calculate the number of rabbits after a few more months?

*1*, *1*, *2*, *3*, *5*, *8*, ,

*,*

*,*

*,*

*,*

*, …*

So after 12 months, you’ll have 144 pairs of rabbits!

This sequence of numbers is called the **Fibonacci Sequence**

When Fibonacci was born in 1175, most people in Europe still used the

When he returned to Italy, Fibonacci wrote a book called *Liber Abaci* (Latin for “The Book of Calculations”), where he first introduced the new Arabic numerals to European merchants. They were an immediate success – and we still use them today.

On one of the pages in his book, he also investigated the breeding patterns of rabbits – that’s why the Fibonacci numbers were named after him.

Of course, the Fibonacci numbers are not how rabbits *actually* populate in real life. Rabbits don’t have exactly one male and one female offspring every single month, and we haven’t accounted for rabbits dying eventually.

But it turns out that there are many other places in nature where Fibonacci numbers *do* appear: for example the spirals in plants. Can you count how many spirals there are in each direction?

This pine cone has

This sunflower has 34 clockwise spirals and 55 counterclockwise spirals.

In both cases, the numbers of spirals are consecutive Fibonacci numbers. The same is true for many other plants: next time you go outside, count the number of petals in a flower or the number of leaves on a stem. Very often you’ll find that they are Fibonacci numbers!

Of course, this is not just a coincidence. There is an important reason why nature likes the Fibonacci sequence, which you’ll learn more about later.

Fibonacci numbers also appear in the populations of honeybees.

In every bee colony there is a single *queen* that lays many eggs. If an egg is fertilised by a male bee, it hatches into a **female** bee. If it is not fertilised, it hatches into a **male** bee (called a drone).

This means that female bees have

If we draw the ancestry tree of a bee, the number of parents, grandparents, great-grandparents, and earlier generations are always Fibonacci numbers!

Occasionally, young female bees are fed with special food called “royal jelly”. In that case, they turn into queens and will fly away to start a new hive.

## The Golden Ratio

Just like the

At every step, the squares form a larger rectangle. Its width and height are always two consecutive Fibonacci numbers. The **aspect ratio** of the rectangle is the ratio of its width and its height:

Notice how, as we add more and more squares, the aspect ratio seems to get closer and closer to a specific number around 1.6. This number is called the **golden ratio**

Many people believe that the golden ratio is particularly aesthetically pleasing. That’s why it is often used by artists and architects – like in these two examples:

We can approximate the golden ratio by

However, it turns out that the exact value of **irrational number**

## Fibonacci Spirals

The golden ratio explains why Fibonacci numbers appear in nature, like the sunflower and pine cone you saw at the beginning of this section.

Both these plants grow outwards from their center (a part of the plant called the *meristem*). As new seeds, leaves or petals are added, they push the existing ones further outwards.

Move the slider on the right to visualise how a plant grows. Notice how every leaf is added at a different rotation than the previous one. The angle between two consecutive leafs is always the same.

It is important for flowers to pick a suitable angle: the leaves or seeds have to be approximately equally spaced so that they get the largest amount of sunlight and nutrients. In the diagram below, you can explore what a sunflower might look like with different angles between its seeds:

*can’t*be closely approximated by a simple fraction.

You might remember from above that the ratios of consecutive Fibonacci numbers get closer and closer to the golden ratio – and that’s why, if you count the number of spirals in a plant, you will often find a Fibonacci number.

It is important to remember that nature doesn’t know about Fibonacci numbers. Nature also can’t solve equations to calculate the golden ratio – but over the course of millions of years, plants had plenty of time to try out different angles and discover the best one.

Plants and animals always want to grow in the most efficient way, and that is why nature is full of regular, mathematical patterns.

## Fibonachos

So far, we have only used the recursive equation for Fibonacci numbers. There actually is an explicit equation, too – but it is much more difficult to find:

We could also try picking different starting points for the Fibonacci numbers. For example, if we start with 2, 1, … rather than 1, 1, … we get a sequence called the **Lucas numbers**.

It turns out that, whatever two starting numbers you pick, the resulting sequences share many properties. For example, the ratios of consecutive terms will *always*

There are many other puzzles, patterns and applications related to Fibonacci numbers. Here are a few examples, which you can try yourself:

### Problem solving

**1. Fibonacci Divisibility**

(a) Which Fibonacci numbers are even? Is there a pattern to where they are positioned along the sequence? Can you explain why?

(b) Which Fibonacci numbers are divisible by 3 (or divisible by 4)? What do you notice?

**2. Fibonacci Sums**

What happens if you add up any three consecutive Fibonacci numbers? Can you explain why?

**3. Fibonacci Staircases**

When walking up the stairs, I can either take single steps or leap over two steps at a time. This means that there are many different possibilities for how I could go up a staircase. For example, if there are 5 steps, I have 8 different choices:

How many choices are there for staircase with 6, 7 or 8 steps? Can you detect a pattern? And how is this related to the Fibonacci numbers?