Prime reciprocal magic square

A prime reciprocal magic square is a magic square using the decimal digits of the reciprocal of a prime number.

Consider a number divided into one, 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... If you examine the multiples of 1/7, you can see that each is a cyclic permutation of these six digits:

1/7 = 0·1 4 2 8 5 7...
2/7 = 0·2 8 5 7 1 4...
3/7 = 0·4 2 8 5 7 1...
4/7 = 0·5 7 1 4 2 8...
5/7 = 0·7 1 4 2 8 5...
6/7 = 0·8 5 7 1 4 2...

If the digits are laid out as a square, each row will sum to 1+4+2+8+5+7, or 27, and only slightly less obvious that each column will also do so, and consequently we have a magic square:

1 4 2 8 5 7
2 8 5 7 1 4
4 2 8 5 7 1
5 7 1 4 2 8
7 1 4 2 8 5
8 5 7 1 4 2

However, neither diagonal sums to 27, but all other prime reciprocals in base ten with maximum period of p-1 produce squares in which all rows and columns sum to the same total.

Other properties of Prime Reciprocals: Midy's theorem

The repeating pattern of an even number of digits [7-1, 11-1, 13-1, 17-1, 19-1, 23-1, 29-1, 47-1, 59-1, 61-1, 73-1, 89-1, 97-1, 101-1, ...] in the quotients when broken in half are the nines-complement of each half:

1/7 = 0.142,857,142,857 ...
     +0.857,142
      ---------
      0.999,999
1/11 = 0.09090,90909 ...
      +0.90909,09090
       -----
       0.99999,99999
1/13 = 0.076,923 076,923 ...
      +0.923,076
       ---------
       0.999,999
1/17 = 0.05882352,94117647
      +0.94117647,05882352
      -------------------
       0.99999999,99999999
1/19 = 0.052631578,947368421 ...
      +0.947368421,052631578
       ----------------------
       0.999999999,999999999

Ekidhikena Purvena From: Bharati Krishna Tirtha's Vedic mathematics#By one more than the one before

Concerning the number of decimal places shifted in the quotient per multiple of 1/19:

01/19 = 0.052631578,947368421
02/19 = 0.1052631578,94736842
04/19 = 0.21052631578,9473684
08/19 = 0.421052631578,947368
16/19 = 0.8421052631578,94736

A factor of 2 in the numerator produces a shift of one decimal place to the right in the quotient.

In the square from 1/19, with maximum period 18 and row-and-column total of 81, 
both diagonals also sum to 81, and this square is therefore fully magic:
01/19 = 0·0 5 2 6 3 1 5 7 8 9 4 7 3 6 8 4 2 1...
02/19 = 0·1 0 5 2 6 3 1 5 7 8 9 4 7 3 6 8 4 2...
03/19 = 0·1 5 7 8 9 4 7 3 6 8 4 2 1 0 5 2 6 3...
04/19 = 0·2 1 0 5 2 6 3 1 5 7 8 9 4 7 3 6 8 4...
05/19 = 0·2 6 3 1 5 7 8 9 4 7 3 6 8 4 2 1 0 5...
06/19 = 0·3 1 5 7 8 9 4 7 3 6 8 4 2 1 0 5 2 6...
07/19 = 0·3 6 8 4 2 1 0 5 2 6 3 1 5 7 8 9 4 7...
08/19 = 0·4 2 1 0 5 2 6 3 1 5 7 8 9 4 7 3 6 8...
09/19 = 0·4 7 3 6 8 4 2 1 0 5 2 6 3 1 5 7 8 9...
10/19 = 0·5 2 6 3 1 5 7 8 9 4 7 3 6 8 4 2 1 0...
11/19 = 0·5 7 8 9 4 7 3 6 8 4 2 1 0 5 2 6 3 1...
12/19 = 0·6 3 1 5 7 8 9 4 7 3 6 8 4 2 1 0 5 2...
13/19 = 0·6 8 4 2 1 0 5 2 6 3 1 5 7 8 9 4 7 3...
14/19 = 0·7 3 6 8 4 2 1 0 5 2 6 3 1 5 7 8 9 4...
15/19 = 0·7 8 9 4 7 3 6 8 4 2 1 0 5 2 6 3 1 5...
16/19 = 0·8 4 2 1 0 5 2 6 3 1 5 7 8 9 4 7 3 6...
17/19 = 0·8 9 4 7 3 6 8 4 2 1 0 5 2 6 3 1 5 7...
18/19 = 0·9 4 7 3 6 8 4 2 1 0 5 2 6 3 1 5 7 8...

The same phenomenon occurs with other primes in other bases, and the following table lists some of them, giving the prime, base, and magic total (derived from the formula base-1 x prime-1 / 2):

PrimeBaseTotal
191081
5312286
5334858
59229
67233
83241
8919792
167685,561
199413,960
19915014,751
2112105
2233222
29314721,316
3075612
383101,719
38936069,646
3975792
42133870,770
48761,215
503420105,169
587368107,531
5933592
6318727,090
677407137,228
757759286,524
787134,716
8113810
9771,222595,848
1,033115,160
1,18713579,462
1,30752,612
1,499117,490
1,8771916,884
1,933146140,070
2,0112625,125
2,02721,013
2,1416366,340
2,53921,269
3,18797152,928
3,3731116,860
3,659126228,625
3,9473567,082
4,26122,130
4,81322,406
5,64775208,902
6,11336,112
6,27723,138
7,28323,641
8,38724,193

See also

References

Rademacher, H. and Toeplitz, O. The Enjoyment of Mathematics: Selections from Mathematics for the Amateur. Princeton, NJ: Princeton University Press, pp. 158–160, 1957.

Weisstein, Eric W. "Midy's Theorem." From MathWorld—A Wolfram Web Resource. http://mathworld.wolfram.com/MidysTheorem.html

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