Numbers spiral diagonals (Part 2) - MATLAB Cody - MATLAB Central

Problem 2342. Numbers spiral diagonals (Part 2)

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Inspired by Project Euler n°28 and 58.
A n x n spiral matrix is obtained by starting with the number 1 and moving to the right in a clockwise direction.
For example with n=5, the spiral matrix is :
21 22 23 24 25
20 7 8 9 10
19 6 1 2 11
18 5 4 3 12
17 16 15 14 13
The sum of the numbers on the diagonals is 101 (See problem 2340) and you have 5 primes (3, 5, 7, 13, 17) out of the 9 numbers lying along both diagonals. So the prime ratio is 5/9 ≈ 55%.
With a 7x7 spiral matrix, the ratio is 62% (8 primes out of the 13 diagonal numbers).
What is the side length (always odd and greater than 1) of the square spiral for which the ratio of primes along both diagonals FIRST falls below p% ? (0<p<1)
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28.81% Correct | 71.19% Incorrect
Last Solution submitted on May 06, 2025

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