More ϕ ( n ) ϕ(n)

Suppose ϕ ( n ) n 1 ϕ(n)|n-1 and that n n is not a prime, then n n may be:

1 - comprised of 2 distinct prime factors

2 - comprised of 3 distinct prime factors

3 - comprised of at least 4 distinct prime factors

Submit your answer as the sum of the squares of the numbers corresponding to the statements which are true, e.g. if statements 2 and 3 are correct then submit the answer as 2 2 + 3 2 2^{2} + 3^{2}

ϕ ( n ) ϕ(n) denotes the Euler-Totient Function

Inspired by George E. Andrews Number Theory


The answer is 9.

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1 solution

This solution is based on the assumption that such a composite n n exists.

1- Obviously n p k m n\neq p^k*m , where k 2 k\geq 2 and p p is a prime and G C D ( p , m ) = 1 GCD(p,m)=1 . Otherwise p ϕ ( n ) p|\phi(n) and we know that p n 1 p \nmid n-1 . Therefore, n n is a square free composite.

2- If n = p q n=pq , where p p and q q are distinct primes, then

( p 1 ) ( q 1 ) p q 1 p 1 q 1 , q 1 p 1 p = q (p-1)(q-1)|pq-1 \implies p-1|q-1 , q-1|p-1 \implies p=q

which suggests that p p and q q are not distinct.

3- If n = p q r n=pqr , where p p , q q and r r are distinct primes and p > q > r p>q>r , then, we need 2 ϕ ( n ) n 2 \phi(n) \leq n and it would be possibly achieved, when p , q , r p,q,r are as small as possible.

ϕ ( n ) = ( p 1 ) ( q 1 ) ( r 1 ) > ( p 1 ) . 4.2 = 8 ( p 1 ) 2.8 ( p 1 ) = 16 ( p 1 ) > 3.5. ( p ) 1 = 15 p 1 p < 16 \phi(n)=(p-1)(q-1)(r-1)> (p-1).4.2=8(p-1) \implies 2.8(p-1)=16(p-1) > 3.5.(p)-1=15p-1 \implies p<16

one can check all the possible p , q , r < 16 p,q,r<16 to realise they won't satisfy the requirement.

4- Note that we need

j = 1 k p j j = 1 k ( p j 1 ) 2 j = 1 k log ( p j ) j = 1 k log ( p j 1 ) j = 1 k 1 p j log 2 \frac{\prod_{j=1}^{k} p_j}{\prod_{j=1}^{k} (p_j-1)} \geq 2 \implies \sum_{j=1}^{k} \log(p_j)- \sum_{j=1}^{k} \log(p_j-1) \approx \sum_{j=1}^{k} \frac{1}{p_j}\geq \log 2

for some k N k \in \mathbb{N} .

j = 1 k 1 p j \sum_{j=1}^{k} \frac{1}{p_j} is a divergent sum, hence, j = 1 k p j j = 1 k ( p j 1 ) 2 \frac{\prod_{j=1}^{k} p_j}{\prod_{j=1}^{k} (p_j-1)} \geq 2 would be satisfied for some k N k \in \mathbb{N} .

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