15 lines
751 B
Markdown
15 lines
751 B
Markdown
Starting with 1 and spiralling anticlockwise in the following way, a square spiral with side length 7 is formed.
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37 36 35 34 33 32 31
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38 17 16 15 14 13 30
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39 18 05 04 03 12 29
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40 19 06 01 02 11 28
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41 20 07 08 09 10 27
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42 21 22 23 24 25 26
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43 44 45 46 47 48 49
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~~~
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It is interesting to note that the odd squares lie along the bottom right diagonal, but what is more interesting is that 8 out of the 13 numbers lying along both diagonals are prime; that is, a ratio of `8/13 = 62%`.
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If one complete new layer is wrapped around the spiral above, a square spiral with side length 9 will be formed. If this process is continued, what is the side length of the square spiral for which the ratio of primes along both diagonals first falls below `10%`? |