Watch him pull a rabbit out of his hat? Not exactly.

Arthur T.
Benjamin eschews the usual trappings of the magician's trade. Calling
himself a mathemagician,
he astonishes audiences with amazing feats of mental arithmetic. Behind the
scenes, he reveals how you, too, can look like a genius without really trying.

Photo by Richard Haverty

A math professor at Harvey Mudd College, Benjamin has
brought his particular brand of prestidigitation to a wide variety of
appreciative audiences. He has appeared on television programs and performed in
night clubs, classrooms, and banquet halls. He has even traded quips with comedian Stephen
Colbert on The
Colbert Report.

Benjamin's act is a striking demonstration of how miraculous
fairly ordinary, simple mathematical machinery can appear when you don't really
know what's going on under the hood.

One of my first encounters—many, many years ago—with
arithmetic shortcuts was in a book titled

*Cheaper by the Dozen*, the story of efficiency expert Frank B. Gilbreth and his family of 12 children. In a chapter about dinner conversation, Frank B. Gilbreth Jr. and Ernestine Gilbreth Carey write, "Also of exceptional general interest was a series of tricks whereby Dad could multiply large numbers in his head, without using pencil and paper."
For example, to multiply 46 times 46, you figure out how
much greater 46 is than 25. The answer is 21. Then you calculate how much less
46 is than 50. The answer is 4. You square 4 and get 16. Putting 16 and 21 together
gives the answer 2116.

That's the sort of venerable procedure that Benjamin uses to
do fast multiplies and to square four-digit numbers faster than someone using a
calculator. Turning a repertoire of such calculating tricks into a real show,
however, requires developing a memory for numbers and learning how to calculate
from left to right, performed at the speed of rapid-fire chatter.

Suppose you want to multiply 378 by 7. Starting from the
left, you would get 2100 (300 x 7) plus something more. In the next step, 70 x
7 equals 490, which is added to 2100 to give 2590, plus something more.
Finally, 7 x 8 equals 56, which is added to 2590 to give 2646.

One advantage of using this method is that you can start
saying the answer while you're still calculating it, Benjamin remarks.

Here's a neat way to square two-digit numbers. Suppose the
number to be squared is 37. That number is 3 less than 40, and 34 is 3 less
than 37. Multiply 40 by 34 to get 1360, then add the square of the difference,
3

^{2}or 9, to get 1369.
The trick is to choose the difference so that the
multiplication is easy. For example, to square 59, choose a difference of 1. Go
up to 60 and down to 58. Multiply 60 times 58 to get 3480, then add 1

^{2}, to obtain 3481.
The proof is in the algebra:

*a*^{2}= (*a*+*d*)(*a*-*d*) +*d*^{2}. The same idea can be extended to squaring 3-digit and 4-digit numbers.
Interestingly, when Benjamin performs calculations involving
long sequences of digits, he relies on a phonetic code to remember the numbers.
"There's no mental blackboard," he says. "It's much more an
auditory process."

Benjamin turns sequences of digits into words that add up to
some sort of crazy scenario. For example, he can mentally convert the sequence
9 6 4 8 3 7 5 4 8 3 1 2 7 5 9 6 into the words "pitcher fume color fume
tinkle beach" as a handy mnemonic for (9 6 4) (8 3) (7 5 4) (8 3) (1 2 7
5) (9 6).

The underlying scheme assigns different consonants to
different numbers, and the memorizer supplies the vowels: 1 (t, d), 2 (n), 3
(m), 4 (r), 5 (l), 6 (j, ch, sh), 7 (c, k, g), 8 (f, v, ph), 9 (p, b), 10 (z,
s). This is the modern version of a number alphabet originally proposed by Pierre
HÃ©rigone and published in Paris in 1634. Three consonants—w, h, and y,
spelling "why"—do not appear in the list.

For more on phonetic codes, see Benjamin’s article "A Better
Way to Memorize Pi: The Phonetic Code," published in the February 2000

*Math Horizons*.
Benjamin can also handle magic squares, natural
logarithms, cube roots, and much more. "As a kid, I liked to show
off," he says. "Now I get paid to do this!" Besides, the
techniques are so easy that even an elementary-school student can master them.

Benjamin describes and explains many of his mental calculation
techniques in the book

*Secrets of Mental Math: The Mathemagician’s Guide to Lightning Calculation and Amazing Math Tricks*(coauthored by Michael Shermer).
He also has a set of DVDs
in The
Great Courses series devoted to "Secrets
of Mental Math." Benjamin’s "Mathemagic" performance, available as a TED video,
has been viewed more than 5 million times.

Benjamin wrote about the mathematics underlying some of his mental
feats in an article published in the January 2012

*College Mathematics Journal*—a special issue dedicated to Martin Gardner. Titled "Squaring, Cubing, and Cube Rooting," the article starts off:
"I still recall the thrill and simultaneous disappointment I
felt when I first read

*Mathematical Carnival*by Martin Gardner. I was thrilled because, as my high school teacher had told me, mathematics was presented there in a playful way that I had never seen before. I was disappointed because Gardner quoted a formula that I thought I had 'invented' a few years earlier. I have always had a passion for mental calculation, and the formula . . . appears in Gardner's chapter on 'lightning calculators.' It was used by the mathematician A. C. Aitken to square large numbers mentally."
Benjamin's article is reprinted in

*Martin Gardner in the Twenty-First Century*, edited by Michael Henle and Brian Hopkins (MAA, 2012).*Original version posted October 5, 1998*

**Reference:**

Gardner, M. 2008. Memorizing numbers. In

*Hexaflexagons, Probability Paradoxes, and the Tower of Hanoi: Martin Gardner's First Book of Mathematical Puzzles and Games*. Cambridge University Press and MAA.
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