# Problem 58: Spiral primes

Starting with 1 and spiralling anticlockwise in the following way, a
square spiral with side length 7 is formed. 37 36 35 34 33 32 31 38 17
16 15 14 13 30 39 18  5  4  3 12 29 40 19  6  1  2 11 28 41 20  7  8  9
10 27 42 21 22 23 24 25 2643 44 45 46 47 48 49 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%. 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%?