Problem 1761

1. Assume for contradiction: Suppose $a \neq b$. Then, $b - a$ has finitely many prime divisors.
2. Lemma I: There exists an odd prime $p > b > a$ such that $4 \nmid a^p + p$ and $\gcd(a^p + p, b - a) \leq 2$.

Proof of Lemma I:
- Let $p$ be an odd prime. If $a$ is even and $p$ is odd, then $4 \nmid a^p + p$.
- If $a \equiv \pm 1 \pmod{4}$, choose $p \equiv a \pmod{4}$. Thus, there exists an odd prime $p$ for which $4 \nmid a^p + p$.
- Now, we prove the second part of Lemma I. Let $q$ be an odd prime divisor of $b - a$. For all $q$, if $a \equiv 1 \pmod{q}$, let $p \equiv 1 \pmod{q}$, then $a^p + p \equiv 2 \pmod{q}$.
- Otherwise, if $a \not\equiv 1 \pmod{q}$, let $p \equiv -1 \pmod{q}$, so $a^p + p \equiv a^p - 1 \pmod{q}$.
- Assume for contradiction that $q \mid a^p - 1$. Then, either $\text{ord}_q(a) = 1$ or $\text{ord}_q(a) = p$. In the former case, this means $a \equiv 1 \pmod{q}$, contradiction. In the latter case, this gives us $q \equiv 1 \pmod{p}$, contradiction, since $p > q$, as $p \equiv -1 \pmod{q}$.
- Hence, by Dirichlet's Theorem of Arithmetic Progression together with the Chinese Remainder Theorem, we find that there exists an odd prime $p > b > a$ such that $\gcd(a^p + p, b - a)$ has no prime factors other than possibly $2$, and since we can choose $p$ such that $4 \nmid a^p + p$, it follows that $\gcd(a^p + p, b - a) \leq 2$, as desired. $\blacksquare$

3. Using Lemma I: Let $n = p$, where $p$ is an odd prime which satisfies the conditions in Lemma I, so we have $a^p + p \mid b^p - a^p$.
4. Prime divisor analysis: Let $q$ be a prime divisor of $a^p + p$. If $q \mid b$, then $q \mid a$, so $q \mid p$, which means that $q = p$. However, $p > a$, therefore we must have $q \nmid a, b$.
5. Order analysis: Then, $q \mid (ba^{-1})^p - 1$, so $\text{ord}_q(ba^{-1}) = 1$ or $\text{ord}_q(ba^{-1}) = p$. In the former case, we obtain $q \mid b - a$, however, since $\gcd(a^p + p, b - a) \leq 2$, this implies $q = 2$. In the latter case, this implies $q \equiv 1 \pmod{p}$, so for all primes $q \mid a^p + p$, either $q = 2$ or $q \equiv 1 \pmod{p}$, and also $4 \nmid a^p + p$, hence we must have either $a^p + p \equiv 1 \pmod{p}$ or $a^p + p \equiv 2 \pmod{p}$, or, equivalently, $a \equiv 1 \pmod{p}$ or $a \equiv 2 \pmod{p}$. Since $p > b > a$, this is only possible if $a = 1$ or $a = 2$.

6. Case analysis:
- If $a = 1$, then if any prime $q \mid b$, setting $n = q - 1$, we obtain $q \mid b^{q - 1} + q - 1 \equiv -1 \pmod{q}$, contradiction. Hence, $b$ cannot have any prime divisors, so $b = 1$.
- If $a = 2$, then $a^p + p$ is odd, so $\gcd(a^p + p, b - a) = \gcd(2^p + p, b - 2) = 1$. Then, we have $a^p + p = 2^p + p \mid b^p - a^p = b^p - 2^p$. Let $q$ be a prime divisor of $2^p + p$. Note that $q$ must be odd. If $q \mid b$, then $q \mid 2$, so $q = 2$, contradiction. As a result, $q \nmid b$, then $q \mid (b \cdot 2^{-1})^p - 1$, therefore $\text{ord}_q(b \cdot 2^{-1}) = 1$ or $\text{ord}_q(b \cdot 2^{-1}) = p$. In the former case, this gives us $q \mid b - 2$, but this is impossible since $\gcd(2^p + p, b - 2) = 1$. In the latter case, this implies $q \equiv 1 \pmod{p}$, so for all primes $q \mid 2^p + p$, $q \equiv 1 \pmod{p}$, so this gives us $2^p + p \equiv 1 \pmod{p}$, but we also have $2^p + p \equiv 2 \pmod{p}$, contradiction.

7. Conclusion: Then, the initial assumption that $b \neq a$ must be wrong, so we find that $b = a$.

$\blacksquare$

The final answer is $\boxed{ a = b }$.