Problems & Puzzles: Conjectures

Conjecture 19.  A bound to the largest prime factor of certain Carmichael numbers

A number n is said to be a "Carmichael number" if:

* x^(n-1)=1 mod n, for all x such that:
* gcd(x, n)=1
* n= an odd composite number having 3 or more prime factors.

It can be shown that the above definition is equivalent to say that for each Carmichael number n, for every prime p that divides n then p-1 divides n-1 also.

From now on, let's discuss only about the C3 or Carmichael odd numbers composed of 3 prime factors such that C3 = n = p*q*r, p<q<r

A few examples are:


At the p.44, of "Recreations in the Theory of Numbers" by Albert H. Beiler we can read:

"if... p is assigned value then there are only a finite number of values for the other two primes [q & r]; they can not be increased indefinitely"

The same idea is said by Eric Weisstein:

"For Carmichael numbers with exactly three prime factors, once one of the primes has been specified, there are only a finite number of Carmichael numbers which can be constructed." (See his article

Then there are only finite values n, if any, for that p value. But then also it's true that for each p, r has some upper limit. 


1) Can you explain the Bailer & Weisstein's claims regarding the finitude of the C3 numbers for a fixed p?

2) How large is r against p?

3) A simple inspection of the first C3 numbers (*) shows that r < p3 (let's take this as a starting conjecture to improve). The same data shows that very soon this bound is too wide... Exist a better (tighter) and simpler bound?


(*) See the car3-18 file from the Richard Pinch's site: 

See other problem related to Carmichael numbers in this site at Problem 24.


The Saturday 6 of May both Richard Pinch and Chris Nash solved the question of ths puzzle-conjecture, basically following the same reasoning and arriving to the same "better bound". The argument, not only the conclusion is interesting. Here are complete: 

Richard Pinch (10:00 am) wrote:

"Let N = pqr be a Carmichael number with three prime factors p < q < r and assume p is given. We have N == 1 mod (p-1), mod (q-1), mod (r-1). Hence qr == 1 mod (p-1), pr == 1 mod (q-1) and pq == 1 mod (r-1). Say pr-1 = a(q-1) and pq-1 = b(r-1).

Since q < r, we have q < r-1 and so p(r-1) > p(r-1)-1 > pq-1 = b(r-1). Hence b < p. Similarly, a(q-1) = pr-1 > p(q-1) so p > a. In fact we see that b=1 is impossible, otherwise we would have pq-1 = r-1, that is, pq=r, but r is prime. Put D = ab - p^2. After a little algebra, we find that q-1 = (p-1)(p+b) / D and r-1 = (p-1)(p+a)/D, which shows that D > 0 and that D = (p-1)(p+b)/(q-1) < p+b < 2p. So there are only a fixed number of possible values of D. For each value of D, there are only finitely many values of a and b, since ab = D+p^2. Hence the are only finitely many pairs (a,b) and hence (q,r) for given p.

Further, we can bound r as follows. a = (D+p^2)/b and a > p. So (p+a)/D < 2a/D < 2(D+p^2)/bD < 2/b + 2p^2/bD <= 1 + p^2. So r-1 < (p^2+1)(p-1) = p^3-p^2+p-1 and so r < p^3-p^2+p < p^3. So (p+a)/D < p/D + (D+p^2)/bD = p/D + 1/b + p^2/bD <= (p^2+2p+1)/2 as b >= 2. So r-1 <= (p-1)(p+1)^2/2 = (p^3+p^2-p-1)/2..."

Chris Nash (1:36 pm) wrote:

"If pqr is a Carmichael number, then the following expressions
I_0=(qr-1)/(p-1) (1)
I_1=(rp-1)/(q-1) (2)
I_2=(pq-1)/(r-1) (3)

must all be integers. In fact, they must all be greater than 1. I_1 is "a little greater" than rp/q. I_2 is "a little greater" than pq/r. I_1*I_2 must also be an integer. I_1*I_2 is "a little greater" than (rp/q)*(pq/r)=p^2. So all you need to prove is that I_1*I_2 is less than p^2+1. (So then we have an integer I_1*I_2 that is between p^2 and p^2+1, which is impossible).

Let p be a known prime, and choose a prime r. We need to find conditions on r to prove a solution does not exist. We proceed by solving for p, q, and r for every possible value of I_2 (all integers greater than one).

Pick I_2, and calculate k such that r=(k.p^3+p-1)/I_2 + 1 Then q=k.p^2+1 by equation (3). By equation (2) I_1 = (rp-1)/(q-1) = (p2+1/k-1/kp)/I_2 + (p-1)/(kp^2) and must be an integer.

Thus I_1.I_2 = p^2 + 1/k - 1/kp + I_2(p-1)/(kp^2) must also be an integer. Since p^2 is an integer, no solution exists if 0< 1/k - 1/kp + I_2(p-1)/(kp^2) < 1

And so no solution exists if k > 1 - 1/p + I_2(p-1)/(p^2) and hence for every possible choice of I_2, there is a limiting value of r, beyond which no solutions exist. Substitute this value of k into the formula for r.

Thus for each I_2, no solution exists for r > (p^3 - p^2 + I_2.p.(p-1) + p - 1)/I_2 + 1 = (p^2 - p + 1) + (p^3 -p^2 + p - 1)/I_2.

This is a decreasing function of I_2. So a tight bound for r is found by putting I_2=2... (then) r<=(p^2-p+1) + (p^3 - p^2 + p - 1)/2 = (p^3 + p^2 -p + 1)/2"

Both arguments reach to the same bound: r<=(p^3+p^2-p+1)/2

But they have obtained some additional things than the originally asked.

When I asked to Richard Pinch if a bound for q against p can be established he replied (2:43 pm) "Yes: I claim q < 2p^2. Using the same notation, one has q-1 = (p-1)(p+b)/D and b < p. So q-1 < (p-1).2p ...."

This means that q<2p^2-2p+1

By his side Chris Nash answered (7/5/2000, 10:12 am) to the same question: "... q<= 2p^2 - 3p +2 and make the comment that this bound cannot be attained unless p=3 ". (Does anybody wants the complete reasoning?)

In this case Richard & Chris have reached to two slightly different results being the Chris's bound a little bit more tight than the Richard's bound  


Regarding the limit of bound for r Chris Nash wrote (1:36 pm):

"...The (limit of the) bound can only be attained if this is prime, and if I_2=2, i.e. if q=p^2+p-1 is also prime and satisfies the conditions. It is not a simpler bound, but it is definitely a tight bound. For instance, if p=3, the tight bound is (27+9-3+1)/2=17. And indeed, 3 *11*17 is a Carmichael number, so this bound is attained. p=5, the bound is (125+25-5+1)/2, which is 73. 5*29*73 is indeed a Carmichael number. The bound is attained yet again... To attain the bound, these three numbers need to be the prime factors: p, p^2+p-1 and (p^3+p^2-p+1)/2... Of course it is unlikely provable that an infinity of such prime triplets exist. Perhaps you would be interested in searching for the largest Carmichael number of this form?"

Later (7/05/2000, 4:19 am), Richard Pinch added:

"I should have added that the proof of the finiteness of qr given p is not original to myself: I think it in Carmichael's paper of 1912, referred to in Beiler's book. I did however give a version of it, similar to the one I sent you, in my paper Carmichael numbers up to 10^15, Math. Comp. 61:203 (Jul 1993) 381--391, which can be found on the net via URL "


So we have two new questions:

a) are there infinite triplets of extreme primes p, p^2+p-1 and (p^3+p^2-p+1)/2
b) find larger and larger of such extreme triplets.


Just as a starting point I tackled the task of finding a kind of moderate large of such  extreme triplets, hoping that readers may soon and happily beat it:

p = 7352265*2^400+1 (is prime)
q = p^2+p-1 (is prime)
r = (p^3+P^2-p+1)/2 (is prime)

Then we have here (Sunday morning) a C3 = p.q.r = an extreme Carmichael C3 number of 764 digits




Records   |  Conjectures  |  Problems  |  Puzzles