Problem 44385. Extra safe primes

Created by Matthew Eicholtz

Solve Later

Did you know that the number 5 is the first safe prime? A safe prime is a prime number that can be expressed as 2p+1, where p is also a prime.

To celebrate Cody's Five-Year Anniversary, write a function to determine if a positive integer n is a safe prime in which the prime p (such that n=2p+1) is also a safe prime.

Examples

isextrasafe(5) = false % because 5=2*2+1 and 2 is not a safe prime
isextrasafe(23) = true % because 23=2*11+1 and 11 is also a safe prime (11=2*5+1)

Solve

Solution Stats

4723 Solutions
753 Solvers

Last Solution submitted on Dec 15, 2025

Last 200 Solutions

Problem Comments

4 Comments

4 Comments

Show 1 older comment

Mehmet OZC on 16 Oct 2017

15 = 2*7+1 and 7 = 2*3+1 so 15 should be a safe prime. Am I missing something?

Peng Liu on 16 Oct 2017

An (extra) safe prime number itself must be a prime number. Here, 15 is not a prime, so it is not (extra) safe prime

danteliujie on 27 Feb 2019

For case 8, x = 719;
which will lead to 44, not prime, the result should be false, but it's true now.
Am I right?

Matthew Eicholtz on 28 Feb 2019

danteliujie, x = 719 does not lead to 44.

Solution Comments

Show comments

Problem Recent Solvers753

Problem Tags

Community Treasure Hunt

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

Start Hunting!

Problem 44385. Extra safe primes

Solution Stats

Last 200 Solutions

Problem Comments

Solution Comments

Problem Recent Solvers753

Suggested Problems

More from this Author43

Problem Tags

Community Treasure Hunt

Problem 44385. Extra safe primes

Solution Stats

Last 200 Solutions

Problem Comments

Solution Comments

Problem Recent Solvers753

Suggested Problems

More from this Author43

Problem Tags

Community Treasure Hunt

Players