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)
Solution Stats
Problem Comments
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4 Comments
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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
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1 Comment
Yuumi Ozaki
on 9 Aug 2019
Why can't 71 be true?
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1 Comment
TurqoiseInt
on 6 Jan 2018
Ups, 'maybe' 15 is not prime at all...
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1 Comment
Francis Naldo
on 29 Dec 2017
Oh sorry. I mean for Test #7
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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.
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1 Comment
Eric Lee
on 17 Dec 2017
isprime already exists!
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1 Comment
yurenchu
on 21 Oct 2017
This code would incorrectly identify x = 59 as an extra-safe prime (which it's not).
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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.
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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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