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Number theory Difficulty 6.4 National olympiad Find the answer

Does there exist positive integers n1,n2,,n2022n_1, n_2, \dots, n_{2022} such that the number
(n12020+n22019)(n22020+n32019)(n20212020+n20222019)(n20222020+n12019) \left( n_1^{2020} + n_2^{2019} \right)\left( n_2^{2020} + n_3^{2019} \right) \cdots \left( n_{2021}^{2020} + n_{2022}^{2019} \right)\left( n_{2022}^{2020} + n_1^{2019} \right)
is a power of 1111?

A number or a short expression. Spacing and $ signs are ignored.

Solution

We are given the expression

(n12020+n22019)(n22020+n32019)(n20212020+n20222019)(n20222020+n12019) \left( n_1^{2020} + n_2^{2019} \right)\left( n_2^{2020} + n_3^{2019} \right) \cdots \left( n_{2021}^{2020} + n_{2022}^{2019} \right)\left( n_{2022}^{2020} + n_1^{2019} \right)

and need to determine if it can be a power of 1111, i.e., 11k11^k for some kNk \in \mathbb{N}.

To approach this problem, we can use modular arithmetic. Consider the expression modulo 1111:

Each term in the product is of the form ni2020+ni+12019 n_{i}^{2020} + n_{i+1}^{2019} . For each positive integer nin_i not divisible by 1111, one of ni2020n_i^{2020}, ni+12019n_{i+1}^{2019} will not necessarily be 0mod110 \mod 11. Note that Fermat's Little Theorem tells us ni101(mod11)n_i^{10} \equiv 1 \pmod{11} (if nin_i is not divisible by 1111), which can extend to show a periodic cycle for the powers involved:

1. ni2020=(ni10)20212021(mod11)n_i^{2020} = (n_i^{10})^{202} \equiv 1^{202} \equiv 1 \pmod{11}.
2. ni2019=ni10×201ni91201ni9ni9(mod11)n_i^{2019} = n_i^{10 \times 201} \cdot n_i^{9} \equiv 1^{201} \cdot n_i^{9} \equiv n_i^{9} \pmod{11}.

Now, observe the expression modulo 1111:
ni2020+ni+120191+ni+19(mod11) n_i^{2020} + n_{i+1}^{2019} \equiv 1 + n_{i+1}^9 \pmod{11}

To be a power of 1111, if the entire product could somehow equate to 0(mod11)0 \pmod{11}, each term would need to individually be divisible by 1111. Otherwise, no complete cancellation or modulus-induced zero can occur. Therefore, each ni2020+ni+120190(mod11)n_i^{2020} + n_{i+1}^{2019} \equiv 0 \pmod{11}.

For ni2020+ni+120190(mod11)n_i^{2020} + n_{i+1}^{2019} \equiv 0 \pmod{11}, we must have:
ni+191(mod11) n_{i+1}^9 \equiv -1 \pmod{11}

The power residue ni+191(mod11)n_{i+1}^9 \equiv -1 \pmod{11} introduces inconsistency because:
- ni9b(mod11)n_i^9 \equiv b \pmod{11} implies that there is no integer that squares to 1-1 (since 1-1 is not a quadratic residue mod 1111).

Thus, by attempt to set ni2020+ni+12019=0(mod11)n_i^{2020} + n_{i+1}^{2019} = 0 \pmod{11} repeatedly contradictions to properties of mod can appear.

Hence, the conclusion is that it is impossible for the given product to be an integer power of 1111 for any choice of positive integers n1,n2,,n2022n_1, n_2, \dots, n_{2022}.

Therefore, the answer is:
No \boxed{\text{No}}

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