All about flooble | fun stuff | Get a free chatterbox | Free JavaScript | Avatars    
perplexus dot info

Home > Just Math
Time for some Prime (Posted on 2013-05-05) Difficulty: 3 of 5
Prove that if m=5^n+3^n+1 is a prime, then 12 divides n.

No Solution Yet Submitted by Danish Ahmed Khan    
No Rating

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution possible solution Comment 1 of 1

The expression is divisible by 3 if n is odd, and divisible by 5 if n is of the form 4n+2. The net effect is that n must be a multiple of 4. But even then, the expression is divisible by 7, unless the said multiple is also divisible by 3.  Hence, if the expression is prime, then 12 divides n.

n=0, n=12, n=36, n=48, are prime solutions.

 n=24 is 103×457×61331×20646607.

 n=60 is 3144853×3847314239×680874654589×105287080263229


  Posted by broll on 2013-05-06 01:29:17
Please log in:
Login:
Password:
Remember me:
Sign up! | Forgot password


Search:
Search body:
Forums (0)
Newest Problems
Random Problem
FAQ | About This Site
Site Statistics
New Comments (23)
Unsolved Problems
Top Rated Problems
This month's top
Most Commented On

Chatterbox:
Copyright © 2002 - 2024 by Animus Pactum Consulting. All rights reserved. Privacy Information