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Singapore Math

Friday, December 13, 2013

13 Math Tidbits on Friday 13

Friday 13 is one of the beloved or lucky days in the Gregorian calendar year for most recreational mathematicians and math bloggers, who can't wait to share some musings about the number 13 and Friday the thirteenth to their non-mathematical friends, fiends, and foes.

1. Triskaidekaphobia Isn't Hereditary

If you can read about the number 13, you probably don't suffer from triskaidekaphobia—the fear of the number 13… but your children and grandchildren still might.

2. Pseudoscience Triumphs Over Profit 

On a plane or in a theater, shouldn't you feel "unsafe" rather than safe because row "13" has often been intentionally mislabeled "14"?

Or, In a lift with no button number 13, would you feel "safer" that they've tried to "cheat death" by making triskaidekaphobes or paraskavedekatriaphobes (who are fearful of Friday the 13th) feel better? All in the name of profit or superstition!

3. Synesthesia AND Autism AND Math

What color and shape is the number 13? What is the taste of 13? 

© [Unknown]

This book may shed some light on the prime number 13!

4. The Closest of Two Friday the 13ths 

Can Friday the 13th occur in two successive months? It's not difficult to see that the shortest span between two consecutive Friday the 13ths is 28 days. How often do you think this event happens? Have you experienced one in your lifetime, or know of someone who rejoices in this kind of numerical (or numerological) oddity?

5. An Unsolved Arithmetic Problem

The number 13 is both a prime and a Fibonacci number. Whether there are infinitely many primes that are also Fibonacci still remains an unsolved problem.

Cracking this higher arithmetic nut may bring you some mathematical prestige or fame, as your proof graces the pages of a Number Theory publication. 

6. A Friday 13 Baby

A baby is born on Friday 13, 2013. When is the next time a baby will be born on the same day of the week in the same month, which also falls on a Friday 13? How often in a 400-year calendrical cycle does this happen?

7. A Friday 13 Survivor 

What are the odds that you will not survive on a Friday the 13th? Is the answer equivalent to finding the probability that a baby will be born on a Friday 13?

8. An Inauspicious Day 

Using the code A = 1, B = 2, C = 3, ...., show that the sum value of the phrase FRIDAY THE THIRTEENTH IS AN UNLUCKY DAY is a multiple of 13.

9. Birth and Death on a Friday 13

What are the chances that one born on a Friday 13 will also die on a Friday 13? How many thousands of folks every year experience this double whammy? 

A solution may be found from Aha! Math.

10. Bio of 13

Prepare a résumé for Number 13, showcasing some of its elementary numerical skills. For instance,

13 = 1 + 4 + 8

13 = 3 + 3¹ + 3²

12 × 13 = 156 and 21 × 31 = 651

13² = 169 and 31² = 961

11. The Unluckiest and the Happiest

List 13 parallelisms between Friday 13 and December 25—what common features the most fearful (or frightful) date of the calendrical year shares with the king of the public holidays. Get creative, by connecting the unconnected!

An ideal gift for numbers lovers & creative problem solvers

12. Social Media 13

Defriend 13 of your Facebook friends, or unfollow 13 of your Twitter followers, who have been inactive in the last 13 months. Send them a Friday 13 alert of your mean intention to save bits and bytes on your RAM to reduce your carbon footprint.

13. Problem Posing on a Friday 13

Pose at least 13 mathematical quickies and trickies, and share them with your on-line friends and fiends.

What is the angle between the hands of a clock at 13:13?

The sum of the ages of Casey and Ian is 13 years. If Casey is 1.3 months older than Ian, how old are they?

A Happy & Blessed Friday 13!


Yan, K. C. (2011). CHRISTmaths: A creative problem solving math book. Singapore: MathPlus Publishing.

Yan, K. C. (2006). Aha! math. Singapore: Panpac Education.

© Yan Kow Cheong, December 13, 2013.