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Problem 659. How long is the longest prime diagonal?

Created by Ned Gulley

Stanislaw Ulam once observed that if the counting numbers are arranged in a spiral, the prime numbers contained in it form a surprising pattern. They appear to cluster along diagonals of the spiral matrix.

Given n, find the longest diagonal arrangement of primes in spiral(n).

Example:

 Input  n = 7
 Output d = 4

Since isprime(spiral(n)) is

     1     0     0     0     1     0     0
     0     0     0     1     0     0     0
     1     0     1     0     0     0     0
     0     1     0     0     1     1     0
     0     0     1     0     1     0     1
     0     1     0     0     0     1     0
     1     0     0     0     0     0     1

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234 correct solutions 208 incorrect solutions
Last solution submitted on Jun 22, 2016

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