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Number theory Difficulty 7.9 National olympiad, round 2 Prove it

For all positive integers nn, show that there exists a positive integer mm such that nn divides 2m+m2^{m} + m.

Proposed by Juhan Aru, Estonia

Solution

1. Define Electric Integers:
Let p1=2<p2=3< p_1 = 2 < p_2 = 3 < \cdots denote the primes in increasing order. Call a positive integer n n electric if there exists kN k \in \mathbb{N} such that
n=p1α1pkαk, n = p_1^{\alpha_1} \cdots p_k^{\alpha_k},
where αi>0 \alpha_i > 0 , and pi1n p_i - 1 \mid n for all i i .

2. Property of Electric Integers:
Note that electric integers have the following property: If mm(modn) m \equiv m' \pmod{n} , and m,mα1 m, m' \geq \alpha_1 , then
2m+m2m+m(modn). 2^m + m \equiv 2^{m'} + m' \pmod{n}.
Furthermore, if n n is electric, then n/pkαk n/p_k^{\alpha_k} is also electric.

3. Induction Hypothesis:
We claim that for any electric integer n n , there exists a positive integer mα1 m \geq \alpha_1 with n2m+m n \mid 2^m + m . We proceed by induction on k k , the number of distinct prime factors of n n .

4. Base Case:
For k=1 k = 1 , n=p1α1 n = p_1^{\alpha_1} . We can choose m=2α1 m = 2^{\alpha_1} , which trivially satisfies n2m+m n \mid 2^m + m .

5. Inductive Step:
Assume the statement is true for k k . Let n=p1α1pkαk n = p_1^{\alpha_1} \cdots p_k^{\alpha_k} be an electric integer. Write n=n/pkαk n' = n/p_k^{\alpha_k} , which is also electric by definition. By the induction hypothesis, there exists mα1 m \geq \alpha_1 such that n2m+m n' \mid 2^m + m .

6. **Constructing mα m_{\alpha} :**
We claim that for all α0 \alpha \geq 0 , we can find mαα1 m_{\alpha} \geq \alpha_1 such that
npkα2mα+mα,mα+1mα(modnpkα). n'p_k^{\alpha} \mid 2^{m_{\alpha}} + m_{\alpha}, \quad m_{\alpha+1} \equiv m_{\alpha} \pmod{n'p_k^{\alpha}}.
We proceed by induction on α \alpha .

7. **Base Case for α \alpha :**
For α=0 \alpha = 0 , set m0 m_0 to be the value that worked for n n' .

8. **Inductive Step for α \alpha :**
Assume it is true for α \alpha . To show it is true for α+1 \alpha + 1 , set mα+1=mα+cnpkα m_{\alpha+1} = m_{\alpha} + c n' p_k^{\alpha} for some choice of c0 c \geq 0 . Note that npkα n' p_k^{\alpha} is electric, so
npkα2mα+1+mα+1. n' p_k^{\alpha} \mid 2^{m_{\alpha+1}} + m_{\alpha+1}.
Furthermore, since mα+1mα(mod(pk1)pkα) m_{\alpha+1} \equiv m_{\alpha} \pmod{(p_k-1)p_k^{\alpha}} , we have 2mα+12mα(modpkα+1) 2^{m_{\alpha+1}} \equiv 2^{m_{\alpha}} \pmod{p_k^{\alpha+1}} .

9. **Choosing c c :**
Pick c c such that
cnpkα2mα+mα(modpkα+1), -c n' p_k^{\alpha} \equiv 2^{m_{\alpha}} + m_{\alpha} \pmod{p_k^{\alpha+1}},
or
cn2mα+mαpkα(modpk). c n' \equiv -\frac{2^{m_{\alpha}} + m_{\alpha}}{p_k^{\alpha}} \pmod{p_k}.
This is possible because pkα2mα+mα p_k^{\alpha} \mid 2^{m_{\alpha}} + m_{\alpha} and gcd(n,pk)=1 \gcd(n', p_k) = 1 .

10. Conclusion:
Since mα+1mα(modn) m_{\alpha+1} \equiv m_{\alpha} \pmod{n'} , we have
2mα+1+mα+10(modn), 2^{m_{\alpha+1}} + m_{\alpha+1} \equiv 0 \pmod{n'},
and also
2mα+1+mα+12mα+mαcnpkα0(modpkα+1). 2^{m_{\alpha+1}} + m_{\alpha+1} \equiv 2^{m_{\alpha}} + m_{\alpha} - c n' p_k^{\alpha} \equiv 0 \pmod{p_k^{\alpha+1}}.
This implies that 2mα+1+mα+1 2^{m_{\alpha+1}} + m_{\alpha+1} is divisible by npkα+1 n' p_k^{\alpha+1} , completing the induction. Note that mα+1mαα1 m_{\alpha+1} \geq m_{\alpha} \geq \alpha_1 .

11. Final Step:
Thus, if n n is electric, we can find m m with 2m+m0(modn) 2^m + m \equiv 0 \pmod{n} . Now, take n=p1β1pkβk n = p_1^{\beta_1} \cdots p_k^{\beta_k} . We set
γi=max(1,βi,νpi(pi+11),νpi(pi+21),,νpi(pk1)) \gamma_i = \max(1, \beta_i, \nu_{p_i}(p_{i+1}-1), \nu_{p_i}(p_{i+2}-1), \ldots, \nu_{p_i}(p_{k}-1))
for 1ik 1 \leq i \leq k . Then N=p1γ1pkγk N = p_1^{\gamma_1} \cdots p_k^{\gamma_k} is electric, and so we can find m m such that
nN2m+m n \mid N \mid 2^m + m
as desired.

\blacksquare

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.