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

Problem 2342. Numbers spiral diagonals (Part 2)

Difficulty:
(1733)
Rate
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)
Solve

Solution Stats

28.65% Correct | 71.35% Incorrect
Last Solution submitted on Mar 20, 2025

Problem Comments

Solution Comments

Show comments

Problem Recent Solvers156

Suggested Problems

More from this Author43

Problem Tags

primes project euler spiral

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!
Go to top of page