Given a fixed positive integer a≥9, we need to prove that there exist finitely many positive integers n satisfying the following conditions:
1. τ(n)=a
2. n∣ϕ(n)+σ(n)
Here, τ(n) is the number of positive divisors of n, ϕ(n) is the Euler's totient function, and σ(n) is the sum of the positive divisors of n.
Assume, for contradiction, that there are infinitely many such n. Let n be expressed in its prime factorized form as n=p1a1p2a2⋯pmam. Given τ(n)=(a1+1)(a2+1)⋯(am+1)=a, which is fixed, we can use the Pigeonhole Principle to assume that m, a1, a2, ..., am are also fixed.
Now, consider the divisibility condition:
n∣ϕ(n)+σ(n).
Substituting the expressions for ϕ(n) and σ(n), we get:
n∣p1a1−1(p1−1)p2a2−1(p2−1)⋯pmam−1(pm−1)+p1−1p1a1+1−1p2−1p2a2+1−1⋯pm−1pmam+1−1.
We need to show that this condition cannot hold for infinitely many n. By induction on m, we start with m=1:
pc∣C1pc−1(p−1)+C2p−1pc+1−1.
This clearly cannot hold for sufficiently large p. Assuming the induction hypothesis for m−1, we need to show it for m. If pi are fixed, we reduce the problem to m−1 and are done. Therefore, pi must get larger and larger.
Considering the limit as pi→∞, we have:
Tn=C1ϕ(n)+C2σ(n).
Dividing and taking the limit to infinity, we get T=C1+C2. Thus,
C1(n−ϕ(n))=C2(σ(n)−n).
Dividing by n, we obtain:
C1(1−p11)⋯(1−pm1)+C2(1+p11+p121+⋯+p1a11)⋯(1+pm1+⋯+pmam1)=C1+C2.
Letting pi→∞, if C1=C2, there will be a contradiction. Therefore, C1=C2, and we get:
(1−p11)⋯(1−pm1)+(1+p11+⋯+p1a11)⋯(1+pm1+⋯+pmam1)=2.
This leads to a contradiction since terms with pq1 cannot be dealt with. Hence, there cannot be infinitely many solutions.
Thus, the answer is: There exist finitely many positive integers } n.}