Multiplicative Magic Square from 9 Terms of Geometric Sequence from 1, 2, 4
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Problem
- With the first $9$ terms of the geometric sequence $1, 2, 4, \ldots$,
- form a product of $4096$ each way.
That is, form a multiplicative magic square whose magic constant is $4096$.
Solution
- $\begin{array}{|c|c|c|}
\hline 2 & 64 & 32 \\ \hline 256 & 16 & 1 \\ \hline 8 & 4 & 128 \\ \hline \end{array}$
Proof
Take the order $3$ (additive) magic square:
- $\begin{array}{|c|c|c|}
\hline 2 & 7 & 6 \\ \hline 9 & 5 & 1 \\ \hline 4 & 3 & 8 \\ \hline \end{array}$
Subtract $1$ from each element:
- $\begin{array}{|c|c|c|}
\hline 1 & 6 & 5 \\ \hline 8 & 4 & 0 \\ \hline 3 & 2 & 7 \\ \hline \end{array}$
This square is still magic.
Now replace each element $n$ with $2^n$:
- $\begin{array}{|c|c|c|}
\hline 2^1 & 2^6 & 2^5 \\ \hline 2^8 & 2^4 & 2^0 \\ \hline 2^3 & 2^2 & 2^7 \\ \hline \end{array}$
which is:
- $\begin{array}{|c|c|c|}
\hline 2 & 64 & 32 \\ \hline 256 & 16 & 1 \\ \hline 8 & 4 & 128 \\ \hline \end{array}$
$\blacksquare$
Sources
- 1821: John Jackson: Rational Amusement for Winter Evenings
- 1992: David Wells: Curious and Interesting Puzzles ... (previous) ... (next): Rational Amusements for Winter Evenings: $152$