# Prime Magic Square/Examples/Order 3/Smallest

## Example of Order $3$ Prime Magic Square

This order $3$ prime magic square has the smallest elements:

$\begin{array}{|c|c|c|} \hline 67 & 1 & 43 \\ \hline 13 & 37 & 61 \\ \hline 31 & 73 & 7 \\ \hline \end{array}$

## Proof

For the purpose of this magic square only, we consider $1$ as a prime.

A simple parity argument can show that $2$ cannot be included in a prime magic square:

If it is, the row containing $2$ sum to an even number, while a row not containing $2$ will sum to an odd number.

We aim to show that all elements of an order $3$ prime magic square has the same remainder when divided by $3$.

There are two parts to this:

### $3$ cannot be used

For simplicity, we denote the numbers in the cells by their remainders when divided by $3$.

Note that $3$ is the only prime divisible by $3$.

We define the off-diagonals as:

$\begin{array}{|c|c|c|} \hline * & & \\ \hline & * & \\ \hline & & * \\ \hline \end{array} \begin{array}{|c|c|c|} \hline & * & \\ \hline & & * \\ \hline * & & \\ \hline \end{array} \begin{array}{|c|c|c|} \hline & & * \\ \hline * & & \\ \hline & * & \\ \hline \end{array} \begin{array}{|c|c|c|} \hline & & * \\ \hline & * & \\ \hline * & & \\ \hline \end{array} \begin{array}{|c|c|c|} \hline & * & \\ \hline * & & \\ \hline & & * \\ \hline \end{array} \begin{array}{|c|c|c|} \hline * & & \\ \hline & & * \\ \hline & * & \\ \hline \end{array}$

We also observe that, by switching rows and columns, the numbers in each row and column remains unchanged, while the two diagonals become two off-diagonals sharing one cell.

Therefore the position of the numbers do not matter in the most part.

Suppose $3$ is used in the square.

Without loss of generality there are only two cases:

#### Case $1$: The row containing $3$ has numbers with all remainders

We have:

$\begin{array}{|c|c|c|} \hline 0 & 1 & 2 \\ \hline & & \\ \hline & & \\ \hline \end{array}$

Hence the row sum is divisible by $3$.

Since:

$1 + 1 \equiv 2 \pmod 3$
$2 + 2 \equiv 1 \pmod 3$
$1 + 2 \equiv 0 \pmod 3$

there is a unique way to fill in the columns:

$\begin{array}{|c|c|c|} \hline 0 & 1 & 2 \\ \hline 1 & 1 & 2 \\ \hline 2 & 1 & 2 \\ \hline \end{array}$

Note that the order of $1$ and $2$ in the leftmost column do not matter due to symmetry.

The sums of rows $2$ and $3$ are not divisible by $3$.

Hence this case cannot occur.

$\Box$

#### Case $2$: The row containing $3$ leave out numbers with some remainder

Without loss of generality suppose $2$ is not used.

Then:

$\begin{array}{|c|c|c|} \hline 0 & 1 & 1 \\ \hline & & \\ \hline & & \\ \hline \end{array}$

Filling in the columns:

$\begin{array}{|c|c|c|} \hline 0 & 1 & 1 \\ \hline 1 & 2 & 2 \\ \hline 1 & 2 & 2 \\ \hline \end{array}$

All off-diagonals sum to $1$, which is not $1 + 1 = 2$.

Hence this case cannot occur.

$\Box$

### Primes of remainder $1, 2$ cannot be mixed

Without loss of generality suppose there are $2$ $1$'s and $1$ $2$.

Then the row sum is not divisible by $3$.

We have:

$\begin{array}{|c|c|c|} \hline 1 & 1 & 2 \\ \hline & & \\ \hline & & \\ \hline \end{array}$

Filling in the first and third columns:

$\begin{array}{|c|c|c|} \hline 1 & 1 & 2 \\ \hline 1 & & 1 \\ \hline 2 & & 1 \\ \hline \end{array}$

Finally, filling up the rows:

$\begin{array}{|c|c|c|} \hline 1 & 1 & 2 \\ \hline 1 & 2 & 1 \\ \hline 2 & 1 & 1 \\ \hline \end{array}$

There must be an off-diagonal with sum divisible by $3$.

Hence this case cannot occur.

$\Box$

Using this result, we divide the primes $\le 73$ into two groups:

Remainder of $1$: $\set {1, 7, 13, 19, 31, 37, 43, 61, 67, 73}$
Remainder of $2$: $\set {5, 11, 17, 23, 29, 41, 47, 53, 59, 71}$

We only need to show these primes cannot form a smaller magic square.

Consider:

$\begin{array}{|c|c|c|} \hline a & b & c \\ \hline d & e & f \\ \hline g & h & i \\ \hline \end{array}$

Let $C$ be the magic constant.

Then:

 $\ds 4C$ $=$ $\ds \paren {a + e + i} + \paren {b + e + h} + \paren {c + e + g} + \paren {d + e + f}$ These are all lines passing through the center $\ds$ $=$ $\ds \paren {a + b + c + d + e + f + g + h + i} + 3 e$ Center counted $4$ times $\ds$ $=$ $\ds 3 C + 3 e$

Hence $e = \dfrac C 3$, which is $\dfrac 1 9$ of the sum of all numbers in the square.

We have:

$1 + 7 + 13 + 19 + 31 + 37 + 43 + 61 + 67 + 73 = 352$
$5 + 11 + 17 + 23 + 29 + 41 + 47 + 53 + 59 + 71 = 356$

$352$ and $356$ have remainders $1$ and $5$ when divided by $9$.

In the lists:

$1, 19, 37, 73$ have a remainder of $1$ when divided by $9$.
$5, 23, 41, 59$ have a remainder of $5$ when divided by $9$.

Omitting each number gives the corresponding center square values:

$39, 37, 35, 31$ for the first list
$39, 37, 35, 33$ for the second list

Only $31, 37$ of the first list are possible candidates.

However:

$73 + 31 > 93$

Hence $31$ fail to produce a magic square.

This leaves $37$, which possibility is demonstrated above.

$\blacksquare$