Maths in a minute: the Fibonacci sequence

Submitted by Rachel on May 1, 2020

Leonardo Fibonacci c1175-1250.

The Fibonacci sequence

$1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...$

is one of the most famous number sequences of them all. We’ve given you the first few numbers here, but what’s the next one in line? It turns out that the answer is simple. Every number in the Fibonacci sequence (starting from $2$ ) is the sum of the two numbers preceding it:

$\displaystyle 2$	$\displaystyle =$	$\displaystyle 1$	$\displaystyle +$	$\displaystyle 1$
$\displaystyle 3$	$\displaystyle =$	$\displaystyle 1$	$\displaystyle +$	$\displaystyle 2$
$\displaystyle 5$	$\displaystyle =$	$\displaystyle 2$	$\displaystyle +$	$\displaystyle 3$
$\displaystyle 8$	$\displaystyle =$	$\displaystyle 3$	$\displaystyle +$	$\displaystyle 5,$

and so on. So it’s pretty easy to figure out that the next number in the sequence above is $55+89 = 144,$ and (in theory at least) to work out all numbers that follow from here to infinity.

Hear the music of the Fibonacci sequence in our podcast!

Where does it come from?

The Fibonacci is named after the mathematician Leonardo Fibonacci who stumbled across it in the 12th century while contemplating a curious problem. Fibonacci started with a pair of fictional and slightly unbelievable baby rabbits, a baby boy rabbit and a baby girl rabbit.

They were fully grown after one month

and did what rabbits do best, so that the next month two more baby rabbits (again a boy and a girl) were born.

The next month these babies were fully grown and the first pair had two more baby rabbits (again, handily a boy and a girl).

Ignoring problems of in-breeding, the next month the two adult pairs each have a pair of baby rabbits and the babies from last month mature.

Fibonacci asked how many rabbits a single pair can produce after a year with this highly unbelievable breeding process (rabbits never die, every month each adult pair produces a mixed pair of baby rabbits who mature the next month). He realised that the number of adult pairs in a given month is the total number of rabbits (both adults and babies) in the previous month. Writing $A_ n$ for the number of adult pairs in the $nth$ month and $R_ n$ for the total number of pairs in the $nth$ month, this gives

$A_ n = R_{n-1}.$

Fibonacci also realised that the number of baby pairs in a given month is the number of adult pairs in the previous month. Writing $B_ n$ for the number of baby pairs in the $nth$ month, this gives

$B_ n = A_{n-1} = R_{n-2}.$

Therefore, the total number of pairs of rabbits (adult+baby) in a particular month is the sum of the total pairs of rabbits in the previous two months:

$R_ n = A_ n + B_ n = R_{n-1}+R_{n-2}.$

Starting with one pair, the sequence we generate is exactly the sequence at the start of this article. And from that we can see that after twelve months there will be $144$ pairs of rabbits.

Where does it go?

Real rabbits don't breed as Fibonacci hypothesised, but his sequence still appears frequently in nature, as it seems to capture some aspect of growth. You can find it, for example, in the turns of natural spirals, in plants, and in the family tree of bees. The sequence is also closely related to a famous number called the golden ratio. To find out more visit our collection of articles about Fibonacci and his mathematics. And if you'd like to hear how the Fibonacci sequence sounds you should listen to our podcast!

About the author

Rachel Thomas is Editor of Plus.

Comments

Maths circulator

Permalink Submitted by jennifer yusi on July 17, 2020

The rabit as a cover of this page, which attracts me to open the post. The number of rabbit is a good way to apply in mathmatics. That's funny!

Starting point

Permalink Submitted by Steve on July 31, 2020

If you start with 0 and 1 then every number after those is the sum of the two numbers preceding it.

History of the Fibonacci Sequence

Permalink Submitted by Shariputra on September 22, 2020

The Fibonacci sequence was actually discovered much earlier by Indian poets and linguists who came across this sequence while studying Sanskrit poetry. More information on this can be found, for example, on the wikipedia page for the Fibonacci numbers: https://en.wikipedia.org/wiki/Fibonacci_number. This topic is also discussed by the Fields Medallist Manjul Bhargava in multiple lectures on YouTube.

Hence, in this article, under the heading 'Where does it come from'?, it might be pertinent to mention that this sequence was in fact discovered much earlier. This might help dispel the misunderstanding that Fibonacci was the first one to come across this sequence. It will also enrich the article by showing the readers how this sequence arose from a completely different (and surprising!) context- poetry!