COMPUTING SCIENCE

# Calculemus!

Celebrating 25 years of celebrating computation

# Perfect Medians

A puzzle attributed to the late David Gale observes that the sequence 1, 2, 3, 4, 5, 6, 7, 8 has a "perfect median," namely 6, because the sum of the terms preceding 6 is equal to the sum of the terms following it. Are there other subsequences of the counting numbers that have perfect medians? For what values of
*
n
*
does the sequence 1, 2, 3,...,
*
n
*
have a perfect median?

You might be able to solve this problem without the aid of a computer, but I made no progress with pencil and paper. On the other hand, a program to search for perfect medians takes only a few lines of code. Instead of checking each sequence 1, 2, 3, ...,
*
n
*
to see if it contains a perfect median, it's easier to turn the problem inside out and check each integer
*
m
*
to see if it is the perfect median of some sequence. The first step is to add up all the numbers less than
*
m
*
; call the result
*
T
*
. (A bright 10-year-old might figure out a way to calculate
*
T
*
without actually doing all the additions.) Then loop through successive numbers starting with
*
m
*
+1, summing as you go. If this sum is ever equal to
*
T
*
, then
*
m
*
is a perfect median.

This approach immediately reveals that 35 is the perfect median of the sequence 1, 2, 3, ..., 49. In a few seconds you discover three more perfect medians: 204, then 1,189 and 6,930. Of course Gale wasn't really asking for a list of numbers; he wanted to know what pattern underlies the numbers. Getting a computer to answer questions of this kind is not so straightforward—unless you do the mathematical equivalent of consulting Google, that is, you look up the numbers in the Online Encyclopedia of Integer Sequences, maintained by Neil J. A. Sloane of AT&T Research. The search retrieves sequence number A001109, along with a wealth of related lore. It turns out that each perfect median
*
m
*
is the square root of a triangular number. In other words,
*
m
*
^{
2
}
dots can be arranged to form either a square or an equilateral triangle. This is a geometric connection I never would have discovered on my own. (For an explanation of the connection between squares, triangles and medians, see the illustration above.)

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