## What Helps You Remember?

*July 6, 2012 at 6:36 am* *
4 comments *

The following joke is terrible, and I must confess that it’s an MJ4MF original. (One wonders why I’d admit that.)

Professor: The polynomial we’ll use will be

x^{3}+ 17x^{2}+ 51x+ 12, but many people find this expression difficult to remember.Student: That’s too bad. Those people should find a new monic.

That joke came to me (the way posing like Tim Tebow just came to Paul Pierce) tonight, when my wife informed me that she’d be returning from London on United Flight 919. “That’ll be easy to remember,” I said. “It’s a palindromic prime, and it’s the eleventh cuban prime.”

A mathematician walks into a tobacco shop and asks for a cigar. The tobacconist pulls one from the case and says, “I think you’ll really like this cigar. It’s from Havana, and I’ve sold 37 of them today.”

“Fantastic!” exclaims the mathematician. “I just love a good cuban!”

(Don’t you feel lucky? That joke is also an MJ4MF original, and this marks the first time that a post on MJ4MF contains more than one original joke. Congratulations for being part of history.)

The problem, of course, is that when I arrive at the airport, I won’t remember the airline, and I’ll remember “eleventh cuban prime” instead of the actual flight number, and it will take me several minutes to realize that (17 + 1)^{3} ‑ 17^{3} = 919 is the eleventh cuban prime. (Note that cuban primes can be expressed in the

form (*x* + 1)^{3} ‑ *x*^{3}, but not every number of the form (*x* + 1)^{3} ‑ *x*^{3} is a prime number.)

While it’s cool that the flight number is the eleventh cuban prime, it may be a completely useless mnemonic.

On the other hand, I used to remember the phone number for my favorite pizza shop (271‑8000) as the concatenation of 3^{3}, 1^{3}, and 20^{3}.

The house in which I grew up had house number 1331, which I remembered as the third row of Pascal’s triangle. (It’s also 11^{3}.)

And I’ll never forget the phone number at my apartment in college: 867‑5309. That’s certainly a memorable number, but what Tommy Tutone may not know is that 8,675,309 is a twin prime (8,675,311 is its partner), and it’s also the hypotenuse of a primitive Pythagorean triple: 8,675,309^{2} = 2,460,260^{2} + 8,319,141^{2}.

There are lots of mnemonics for remembering myriad things, both mathy and not:

- PEMDAS (order of operations)
- SOHCAHTOA (trig relationships)
- HOMES, or Super Man Helps Every One (Great Lakes)
- Every Good Boy Deserves Fudge (notes on the musical scale)
- ROY G. BIV (colors of the spectrum)
- Spring Forward, Fall Back (setting your clock for daylight savings)
- Lo d(Hi) Less Hi d(Lo), Draw the Line and Square Below (derivative of a quotient)

I think that mnemonics used to remember numbers are more interesting than those used to remember other things, though, primarily because they are often individually created and very personal; that is, they’re used to remember a specific address, phone number, or other piece of information that’s only pertinent to you.

So, tell me — **what’s your best mnemonic**, and what number does it help you remember?

Entry filed under: Uncategorized. Tags: cigar, cuban prime, mnemonic, monic.

1.xander | July 6, 2012 at 9:40 amI am nearly done with a masters degree in mathematics, and it wasn’t until last spring, when I mentioned to a friend that I was studying for the math subject GRE, that I encountered the Lo d(Hi) Hi d(Lo) mnemonic. I feel that I have missed out. 😦

In other news, my daughter was born on the 21st of October, which has the nice property that the prime factors of her birthday add to her birth month.

2.venneblock | July 6, 2012 at 10:33 amNice! My sons were born on 05/02/07, and 5 + 2 = 7. (Just yesterday, Eli said to me, “Today is a special day.” When I asked why, he pointed out that 7 + 5 = 12.)

3.Kimberley Girard | July 6, 2012 at 11:59 amAs an alternative to sohcahtoa, an exchange student from Australia told me she learned it as ohsahcoat. In either case, my students who were poor spellers learned it made a difference if you spelled this one wrong!

4.venneblock | July 6, 2012 at 1:45 pmTo remember SOHCOHTOA, a friend’s high school teacher taught them Sex Over Hot Coals Adds Heat To Ordinary Affection. Oish.