## Happy e day!

Today is February 7, 2018. If you write this the American way, with the month first, and leave out the 20, this gives 2/7/18. And 2, 7, 1 and 8 are the first four digits of one of the most important numbers in mathematics,

This is why today we celebrate day.

To help get the party started, here are some of our favourite *Plus* articles about . But before you delve into them, why not find your birthday within the decimal expansion of with this little gadget produced by the wonderful people of Imaginary. Enjoy!

Maths in a minute: Compound interest and *e* — Compound interest is the curse of debt and the blessing of saving. Find out how it works and what it has to do with

Maths in a minute: Euler's identity —Here's a quick introduction to the beauty queen amongst mathematical formulas, which involves .

The making of the logarithm —The natural logarithm is intimately related to the number and that's how we learn about it at school. When it was first invented, though, people hadn't even heard of and they weren't thinking about exponentiation either. How is that possible?

Have we caught your interest? —This is a longer and more detailed article about compound interest and , complete with some history.

Polar power —Like spirals and flowers? Then you'll love polar coordinates and the pretty pictures they allow you to draw. One of those is intimately related to .

Radioactive decay and exponential laws —Arguably, the exponential function crops up more than any other when using mathematics to describe the physical world. This article looks at radioactive decay and exponential laws.

Light attenuation and exponential laws —This is another article that explores the appearance of exponential laws in nature. It explores light attenuation: the way in which light decreases in intensity as it passes through a medium. the a