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Prime Reciprocal Magic SquareIn mathematics, a reciprocal is 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: 03333... However, the remainders of 1/7 repeat over six, or 7-1, digits: 1/7 = 0142857142857142857... 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, it is obvious that 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. 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): | Prime | Base | Total | | 19 | 10 | 81 | | 53 | 12 | 286 | | 53 | 34 | 858 | | 59 | 2 | 29 | | 67 | 2 | 33 | | 83 | 2 | 41 | | 89 | 19 | 792 | | 167 | 68 | 5,561 | | 199 | 41 | 3,960 | | 199 | 150 | 14,751 | | 211 | 2 | 105 | | 223 | 3 | 222 | | 293 | 147 | 21,316 | | 307 | 5 | 612 | | 383 | 10 | 1,719 | | 389 | 360 | 69,646 | | 397 | 5 | 792 | | 421 | 338 | 70,770 | | 487 | 6 | 1,215 | | 503 | 420 | 105,169 | | 587 | 368 | 107,531 | | 593 | 3 | 592 | | 631 | 87 | 27,090 | | 677 | 407 | 137,228 | | 757 | 759 | 286,524 | | 787 | 13 | 4,716 | | 811 | 3 | 810 | | 977 | 1,222 | 595,848 | | 1,033 | 11 | 5,160 | | 1,187 | 135 | 79,462 | | 1,307 | 5 | 2,612 | | 1,499 | 11 | 7,490 | | 1,877 | 19 | 16,884 | | 1,933 | 146 | 140,070 | | 2,011 | 26 | 25,125 | | 2,027 | 2 | 1,013 | | 2,141 | 63 | 66,340 | | 2,539 | 2 | 1,269 | | 3,187 | 97 | 152,928 | | 3,373 | 11 | 16,860 | | 3,659 | 126 | 228,625 | | 3,947 | 35 | 67,082 | | 4,261 | 2 | 2,130 | | 4,813 | 2 | 2,406 | | 5,647 | 75 | 208,902 | | 6,113 | 3 | 6,112 | | 6,277 | 2 | 3,138 | | 7,283 | 2 | 3,641 | | 8,387 | 2 | 4,193 |
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