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Old 2017-01-16, 22:53   #1
Jan 2017

116 Posts
Default phi function

can any one help

prove that the integer n>1 i s prime if and only if

phi(n) > n - (n)^1/2

where phi is Euler's phi function
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Old 2017-01-17, 03:49   #2
axn's Avatar
Jun 2003

5,179 Posts

This is incorrect. The correct condition is phi(n) = n-1

EDIT:- Or not. I suppose that condition works too. HINT. Assume that n is composite. What is the least number of integers that have a common factor with n.

Last fiddled with by axn on 2017-01-17 at 03:58 Reason: thinking...
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Old 2017-01-17, 09:56   #3
Nick's Avatar
Dec 2012
The Netherlands

2·3·293 Posts

Here is a little more detail to axn's hint.

If n is prime then \(\phi(n)=n-1>n-\sqrt{n}\) since n>1.

If n is not prime then n=km for some integers k,m with 1<k<n and 1<m<n.
At least one of k and m is greater than or equal to \(\sqrt{n}\) and we may choose the names so that \(k\geq\sqrt{n}\).
Now \(\phi(n)\) equals the number of integers in the list 0,1,2,,...,n-1 which have no factor in common with n.
How many numbers in the list are multiples of m?
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Old 2017-01-18, 01:41   #4
MattcAnderson's Avatar
"Matthew Anderson"
Dec 2010
Oregon, USA

953 Posts

Hopefully this helps with understanding of Euler phi function.

The Online Encyclopedia of Integer Sequences lists values of phi(n).

Also, I like Wolfram as a reference

For example phi(6) = 2 because 1 and 5 are relatively prime to 6.

For what its worth.
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