Dear Madam and, “We are now going to talk about sex.”

I collect the applications from our Research Experiences for Undergraduates program.

The most recent starts out, “Dear Madam.” I am annoyed. I made sure that my title, Dr., and name was given clearly on the web page with application instructions. Send to Dr. So-and-so at this address.

Use my proper academic title. Why is this so hard?


One of my students was falling asleep in class today. Fortunately, we were discussing a mathematical model for a rabbit population, and so I could truthfully say, “We are now going to talk about sex.” That did the trick.

Can you guess who formulated this model? Here are the assumptions:

  1. We start with one pair of baby rabbits, 1 male, 1 female.
  2. Rabbits take 1 month to reach sexual maturity.
    (We were discussing the previous step as the student’s eyes closed.)
  3. As soon as rabbits reach sexual maturity they mate. The female gives birth to a male, female pair the next month, and every month thereafter.
  4. Rabbits never die. At least not for the duration of this modeling scenario.

So how many rabbits are there?

  • The first month, there is one pair of rabbits.
  • The second month, the rabbits reach maturity and mate. There is still one pair of rabbits.
  • The third month, the rabbits give birth to another pair and get pregnant again. There are two pairs of rabbits.
  • The fourth month, the first pair gives birth again, and the first two pairs get pregnant. There are three pairs of rabbits.
  • The fifth month, the first two pairs give birth, and the third pair gets pregnant for the first time. There are five pairs of rabbits.
  • As you can see, each month the number of births we get equals the number of rabbits 2 months back. We add that to the rabbits we had in the previous month. So the number of rabbits equals the sum of the preceding two numbers.

Can you identify who created this mathematical model and when it was created?
One option to check your answer, and
here is a second option to check your answer.