Problems & Puzzles: Puzzles

 Puzzle 628. Fermat Pseudoprimes Fred Schneider sent the following nice puzzle: Are there any composite Fermat pseudoprimes n, such that:  n= x^3y.   x > 1, y can be positive integer (including a multiple of x)?

Contributions came from Carlos Rivera, Emmanuel Vantieghem & Jan van Delden

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Carlos wrote:

By simple inspection of a Table in the web I found that 112 = 2^3*14 is a Fermat pseudo prime for the base 65(See http://en.wikipedia.org/wiki/Fermat_pseudoprime).

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Emmanuel wrote:

It is not difficult to show that, if  n  is a product of two relatively prime odd numbers  A, B   (> 1), then  n  is a composite Fermat pseudoprime.

Indeed, let  b  be a positive integer such that  b === 1 (mod A)  and  b === -1 (mod B)  (the existence of  b  is ensured by the Chinese Remainder theorem).  Clearly, b  is not  1  or  n-1  modulo  n.  Further, b^2 === 1 (mod AB)  and hence :

b^(n-1) = (b^2)^((n-1)/2) === 1 (mod n).

Thus,  n  is Fermat pseudoprime to the base  b.

If  n  = p^a (p > 3, a > 1), then the multiplicative group of the integers modulo  n  is cyclic of order  (Euler)phi(n) = (p-1)*(p^(a-1)) and thus, there exist numbers  b  with the property

b^(p-1) === 1 (mod n)  (that not all of these numbers are  1  or  -1 (mod n)  is due to the fact that  p > 3).  It is clear that for such numbers we also have  b^(p^a - 1) === 1 (mod n).  Thus, those  b  are basis for the pseudoprimality of  n.

It is easy to show that no pseudoprime of the form  3^a (a > 1) exists.

To find an even pseudoprime of the form  8k , table at https://oeis.org/A039772  gives  n = 112 as smallest pseudoprime (basis = 65 or 81).

If the basis must be  2, then we can prove the following :

If  n  is a base 2 pseudoprime of the form  y*(x^3), then x  is divisible by a Wieferich prime.

Proof : Let  n = y*(x^3)  be a base 2 pseudoprime.

Let  p  be a prime dividing  x.  Let  r  be the multiplicative order of  2 (mod p^3).  This means that  r  is the smallest positive number such that  2^r === 1 (mod p^3)  and hence that every  m  with the property  2^m === 1 (mod p^3)  is a multiple of  r.

From 2^((p-1)*p^2) === 1 (mod p^3) (Little Fermat), it follows that  r  divides  (p-1)*p^2.

From  2^(n-1) ===1 (mod n), it follows  2^(n-1) ===1 (mod p^3), and hence  r  divides  n-1.

That means that  r  cannot be divisible by  p  and thus must be a divisor of  p-1.

Thus, 2^(p-1) === 1 (mod p^3)  and hence

2^(p-1) === 1 (mod p^2),

which means that  p  is a Wieferich prime, QED.

So, there is an infinitesimal chance that some puzzler ever will find an example of such  n.  The only two known Wieferich primes are  p= 1093  and  p = 3511  and in both cases we have     2^(p-1) !=== 1 (mod p^3).

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Jan wrote:

If p^r|n, p prime, and n is psp(a) then a^(p-1)=1 mod (p^r). [Necessary condition, not sufficient!]

If a=2 only a few solutions are known if r>1, all have r=2, for instance n=1093^2, 3511^2, 4733.1093^2.
If a>2 there are an infinite number of solutions if no further restrictions are imposed on a.
For instance if a=n+1 we always have a solution. If a=n-1 and n is odd we always have a solution.
So I would suggest to impose 1<a<n-1, in accordance with Pomerance, Selfridge and Wagstaff.

If we apply the condition to n=p^r.q^s we search one value of a for which a^(p-1)=1 mod(p^r) and a^(q-1)=1 mod (q^s).
Suppose the first has solution a1 then a=a1 mod(p^r) is also a solution. Similarly a=a2 mod(q^s) for the second equation.
With the Chinese Remainder Theorem we can find the minimal solutions a mod(n), these might very well be smaller than n-1:

If we have n=3^3.5 we could use a=+/-1 mod 27 and a=+/-1,2 mod 5, leading to a equal to either 26 or 109 mod(n).
Testing n=3^r.5 gives solutions r=2 mod 4 and r=3 mod 4 of the form a=3^r-1.
Here r=2 corresponds to,a=3 mod 5 the second to a=1 mod 5. And since 3^4=1 mod 5 the others are solutions too.

So more generally: if n=p^r.q and p and q are odd we (at least) could try a=p^r-1. The corresponding modulus seems to be determined by the order of p mod q.

n=5^r.3 gives a=5^r-1 with r=1 mod 2.
n=3^r.7 gives a=3^r-1 with r=2 mod 6
n=7^r.3 no solutions of the form a=7^r-1
n=5^r.7 gives a=5^r-1 with r=2 mod 6 or r=4 mod 6
n=7^r.5 gives a=7^r-1 with r=1 mod 4 or r=2 mod 4

However I also found:

n=3^r.11 with a=3^r-1 with r=4 mod 4 and 4 is not a divisor of 10! So there is a mistake in my reasoning.

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