Appreciation

Today we celebrate a Derangement Day!  Usually I call a day like today a Permutation Day because the digits of the day and month can be rearranged to form the year, but there’s something extra special about today’s date:

The numbers of the month and day are a derangement of the year:  that is, they are a permutation of the digits of the year in which no digit remains in its original place!

Derangements pop up in some interesting places, and are connected to many rich mathematical ideas.   The question “How many derangements of n objects are there?” is a fun and classic application of the principle of inclusion-exclusion.  Derangements also figure in to some calculations of e and rook polynomials.

So enjoy Derangement Day!  Today, it’s ok to be totally out of order.

Today we celebrate a Permutation Day!  I call days like today permutation days because the digits of the day and the month can be rearranged to form the year.

This is our third Permutation Day of the year, and we’ve got a few more coming up.

Celebrate Permutation Day by mixing things up!  Try doing things in a different order today.  Just remember, for some operations, order definitely matters!

Happy Square Day!  It’s quite rare for the numbers of the date to all be perfect squares, and even rarer for them to be consecutive!

April, 2016 has been a fun month for mathematical dates!  So far this month we’ve had 4/2/16 (an Exponent Day), 4/4/16, and 4/8/16 (a Geometric Mean Day, or alternately, a Powers of 2 Day).  And there are more fun mathematical dates to come!