Problem 44385. Extra safe primes

Created by Matthew Eicholtz

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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

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3774 Solutions
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Last Solution submitted on Jan 27, 2020

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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

Solution 1895606

1 Comment

1 Comment

Yuumi Ozaki on 9 Aug 2019

Why can't 71 be true?

Solution 1407391

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1 Comment

TurqoiseInt on 6 Jan 2018

Ups, 'maybe' 15 is not prime at all...

Solution 1397203

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1 Comment

Francis Naldo on 29 Dec 2017

Oh sorry. I mean for Test #7

Solution 1397200

2 Comments

2 Comments

Francis Naldo on 29 Dec 2017

Test #5 x = 15;
15 = 2*7 + 1
7 = 2*3 + 1
3 is a prime number. therefore 7 is a safe prime. AND 15 is an extra safe prime. Or is there anything wrong with the equations above?

Francis Naldo on 29 Dec 2017

Lol. Forgot that 15 is not even a prime number.

Solution 1317340

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1 Comment

Eric Lee on 17 Dec 2017

isprime already exists!

Solution 1304540

1 Comment

1 Comment

yurenchu on 21 Oct 2017

This code would incorrectly identify x = 59 as an extra-safe prime (which it's not).

Solution 1300442

2 Comments

2 Comments

Li jw on 19 Oct 2017

When x = 5 and x = 7, return false, is it right?

Matthew Eicholtz on 19 Oct 2017

Yes, x = 5 and x = 7 are safe primes, but not "extra" safe primes because their corresponding values for p (2 and 3, respectively) are not safe primes.

Solution 1293071

2 Comments

2 Comments

yurenchu on 18 Oct 2017

This solution would incorrectly identify x = 71 as an extra-safe prime (which it's not).

Matthew Eicholtz on 19 Oct 2017

Good catch, yurenchu. I'm adding 71 to the test suite.

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Problem 44385. Extra safe primes

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Problem 44385. Extra safe primes

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