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# Chinese remainder theorem

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The chinese **remainder theorem** is a theorem from number theory. It is about congruence. The original form was:

*How many soldiers are there in Han Xin's army? – If you let them parade in rows of 3 soldiers, two soldiers will be left. If you let them parade in rows of 5, 3 will be left, and in rows of 7, 2 will be left.*

The theorem says that there will be a solution to this question if there's no common factor between the row sizes. Using the original example, that is that no number divides both 3 and 7, both 3 and 5, nor both 5 and 7 (except, of course, 1). They're all coprime.

The Chinese remainder theorem is used in cryptography. For example, for the RSA algorithm.

## Other websites

- Chinese remainder theorem at cut-the-knot
- "Chinese Remainder Theorem" by Ed Pegg, Jr., The Wolfram Demonstrations Project, 2007