1. Define Electric Integers:
Let p1=2<p2=3<⋯ denote the primes in increasing order. Call a positive integer n electric if there exists k∈N such that
n=p1α1⋯pkαk,
where αi>0, and pi−1∣n for all i.
2. Property of Electric Integers:
Note that electric integers have the following property: If m≡m′(modn), and m,m′≥α1, then
2m+m≡2m′+m′(modn).
Furthermore, if n is electric, then n/pkαk is also electric.
3. Induction Hypothesis:
We claim that for any electric integer n, there exists a positive integer m≥α1 with n∣2m+m. We proceed by induction on k, the number of distinct prime factors of n.
4. Base Case:
For k=1, n=p1α1. We can choose m=2α1, which trivially satisfies n∣2m+m.
5. Inductive Step:
Assume the statement is true for k. Let n=p1α1⋯pkαk be an electric integer. Write n′=n/pkαk, which is also electric by definition. By the induction hypothesis, there exists m≥α1 such that n′∣2m+m.
6. **Constructing mα:**
We claim that for all α≥0, we can find mα≥α1 such that
n′pkα∣2mα+mα,mα+1≡mα(modn′pkα).
We proceed by induction on α.
7. **Base Case for α:**
For α=0, set m0 to be the value that worked for n′.
8. **Inductive Step for α:**
Assume it is true for α. To show it is true for α+1, set mα+1=mα+cn′pkα for some choice of c≥0. Note that n′pkα is electric, so
n′pkα∣2mα+1+mα+1.
Furthermore, since mα+1≡mα(mod(pk−1)pkα), we have 2mα+1≡2mα(modpkα+1).
9. **Choosing c:**
Pick c such that
−cn′pkα≡2mα+mα(modpkα+1),
or
cn′≡−pkα2mα+mα(modpk).
This is possible because pkα∣2mα+mα and gcd(n′,pk)=1.
10. Conclusion:
Since mα+1≡mα(modn′), we have
2mα+1+mα+1≡0(modn′),
and also
2mα+1+mα+1≡2mα+mα−cn′pkα≡0(modpkα+1).
This implies that 2mα+1+mα+1 is divisible by n′pkα+1, completing the induction. Note that mα+1≥mα≥α1.
11. Final Step:
Thus, if n is electric, we can find m with 2m+m≡0(modn). Now, take n=p1β1⋯pkβk. We set
γi=max(1,βi,νpi(pi+1−1),νpi(pi+2−1),…,νpi(pk−1))
for 1≤i≤k. Then N=p1γ1⋯pkγk is electric, and so we can find m such that
n∣N∣2m+m
as desired.
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