A prime reciprocal magic square is a magic square using the decimal digits of the reciprocal of a prime number.
Introduction Consider a unit fraction, like 1/3 or 1/7. In base ten, the remainder, and so the digits, of 1/3 repeats at once: 0.3333.... However, the remainders of 1/7 repeat over six, or 7−1, digits: 1/7 = 0·142857142857142857... Examining the multiples of 1/7, each is a cyclic permutation of these six digits: 1 / 7 = 0.142857 … 2 / 7 = 0.285714 … 3 / 7 = 0.428571 … 4 / 7 = 0.571428 … 5 / 7 = 0.714285 … 6 / 7 = 0.857142 … {\displaystyle {\begin{aligned}1/7&=0.142857\dots \\2/7&=0.285714\dots \\3/7&=0.428571\dots \\4/7&=0.571428\dots \\5/7&=0.714285\dots \\6/7&=0.857142\dots \end{aligned}}}
If the digits are laid out as a square, each row and column sums to 1 + 4 + 2 + 8 + 5 + 7 = 27. This yields the smallest base-10 non-normal, prime reciprocal magic square:
In contrast with its rows and columns, the diagonals of this square do not sum to 27; however, their mean is 27, as one diagonal adds to 23 while the other adds to 31. All prime reciprocals in any base with a p − 1 {\displaystyle p-1} period will generate magic squares where all rows and columns produce a magic constant, and only a select few will be full, such that their diagonals, rows and columns collectively yield equal sums.
Decimal expansions In a full, or otherwise prime reciprocal magic square with p − 1 {\displaystyle p-1} period, the even number of k−th rows in the square are arranged by multiples of 1 / p {\displaystyle 1/p} — not necessarily consecutively — where a magic constant can be obtained. For instance, an even repeating cycle from an odd, prime reciprocal of p that is divided into n−digit strings creates pairs of complementary sequences of digits that yield strings of nines (9) when added together:
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