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Chinese Remainder Theorem for moduli that are not necessarily pairwise coprime

Mathematics Asked on December 23, 2021

Assume that ${n_1},cdots ,{n_k}$ is a finite set of $k$ positive integers. We are not assuming that these integers are pairwise coprime. Consider the homomorphism of additive groups ${phi}:{bf Z}rightarrow {{bf Z}_{n_1}}times cdots times {{bf Z}_{n_k}}$ defined by $${phi}(x) = (xmod {n_1},cdots ,xmod{n_k}).$$
Is this mapping always surjective? If the moduli are pairwise coprime then we can use the Chinese Remainder Theorem to show that it must be surjective. But is this true in general?

One Answer

No consider the map $$mathbb{Z} to mathbb{Z}/2mathbb{Z} times mathbb{Z}/2mathbb{Z}$$

Then the element $(0, 1)$ for example does not get hit.

My question to you, can you generalise this to show that the map is never surjective when we do not have coprimality?

Answered by Mummy the turkey on December 23, 2021

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