Yesterday was 23/11, also known in some parts as 11/23, and you may recognise this as being a date made of the first four Fibonacci numbers. (Such numerical date-based Fibonacci coincidences haven’t been as exciting since 5/8/13, but at least this is one we can celebrate annually.) This meant that mathematicians everywhere got excited about #FibonacciDay, and spent the day talking about the amazing sequence. Here’s a round-up of some of the best bits, so you can celebrate Fibonacci day in style.

First, you’ll need to educate yourself. There are some fab videos on the topic – here’s one by Art Benjamin, and the first in a series of three by Vi Hart. There’s also an article in Plus Magazine, by Ron Knott.

If you prefer a less serious angle, you could try this FoxTrot comic strip on the subject, or keep an eye out for Fibonacci pigeons (here presented with some satisfyingly rigorous analysis).

To celebrate Fibonacci Day properly, you’ll need to decorate the place – you could get this Fibonacci lamp, in the shape of a Golden Spiral, or go more serious and build Fibonacci numbers into the structure of your building. Or, like MIT, go for some funky wall art:

With golden spirals on our walls, every day is #FibonacciDay at MIT. #onlyatMIT #aroundMIT pic.twitter.com/ZFHwKi2MuK

— MIT (@MIT) November 23, 2014

For a delicious Fibonacci Day snack, why not make Golden Ratio Battenberg cake (see above)? And to wash it down, perhaps some of Andrea Hawksley’s Fibonacci Lemonade (link currently down, but a description and photo are here), with ingredients in the appropriate proportions? While you eat, talk about things attributed to Leonardo De Fibonacci other than the ubiquitous numbers. Here’s a suggestion from Evelyn Lamb:

I guess someone decided today is #FibonacciDay? Here’s what you should celebrate: Fibonacci bringing the Hindu-Arabic numerals to Europe! — Evelyn Lamb (@evelynjlamb) November 23, 2014

You can read more about the man himself in this piece about Keith Devlin’s book, The Man of Numbers: Fibonacci’s Arithmetic Revolution. Or you can get the book.

People also made sure they spent the correct proportion of the day pointing out that not everything you hear about the Golden Ratio is true – internet maths person, Vi Hart, got animated about it (attracting at least one hilarious response):

The nautilus shell is not a golden spiral. The parthenon is not the golden ratio. The Mona Lisa has no golden rectangles. #FibonacciDay

— Vi Hart (@vihartvihart) November 23, 2014

The Pyramids are not golden triangles. Rabbits don’t reproduce in fibonacci numbers. The galaxy is not a golden spiral. #FibonacciDay — Vi Hart (@vihartvihart) November 23, 2014

Most rectangles aren’t golden rectangles. Most spirals aren’t golden spirals. Most exponential growth isn’t fibonacci growth. #FibonacciDay — Vi Hart (@vihartvihart) November 23, 2014

The nautilus shell has NOTHING to do with Fibonacci. Found 10 nautilus covers in Powell’s math section. #FibonacciDay pic.twitter.com/iOMmkA074I — Vi Hart (@vihartvihart) November 23, 2014

If you too are extremely bored with Fibonacci/Golden Ratio things illustrated with photos of an unrelated shell, Patrick Honner recommends a video by George Hart, in which he explains what a nautilus shell would look like if it were really a golden spiral – and of course, he’s 3D-printed one.

The inverse of the golden ratio, minus 1, is itself! An amazing magic property? No, it’s THE DEFINITION OF THE NUMBER. Humans made it up.

— Vi Hart (@vihartvihart) November 23, 2014

Also, don’t get me started on people who think that A4 paper is in the Golden Ratio – recent TV gaffes on this have included Tom Dyckhoff, in his documentary The Secret Life of Buildings, and on BBC nerd quiz Only Connect a few weeks ago. Here’s another nice blog post, about the Golden Ratio jumping the shark.

A longish post by Donald E. Simanek rounds up some of the ways in which the Golden Ratio is cool, and some others in which people say it is but it’s actually not. Meanwhile, Edmund Harriss recommends an old MAA piece by Prof. Keith Devlin, on Golden Ratio myths.

I used to have trouble spelling “Fibbonaci” or is it “Fibonnaci” or “Fibbonacci”? Then I noticed that the first 3 numbers in the sequence 1,1,2 correspond to the number of b’s, n’s and c’s in the correct spelling: Fibonacci.

Here’s a knitting pattern for next year’s Fibonacci day – little tree designed by Julia Collins for Botanica Mathematica.

https://botanicamathematica.wordpress.com/2014/04/01/fibonacci-tree/