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Everything about Linear Congruence Theorem totally explained

In modular arithmetic, the question of when a linear congruence can be solved is answered by the linear congruence theorem. If a and b are any integers and n is a positive integer, then the congruence:axb (mod n)      (1) has a solution for x if and only if b is divisible by the greatest common divisor d of a and n (denoted by gcd(a,n) | b). When this is the case, and x0 is one solution of (1), then the set of all solutions is given by »

System of linear congruences

By repeatedly using the linear congruence theorem, one can also solve systems of linear congruences, as in the following example: find all numbers x such that » 2x ≡ 2 (mod 6)


   3x ≡ 2 (mod 7) » 2x ≡ 4 (mod 8)

By solving the first congruence using the method explained above, we find x ≡ 1 (mod 3), which can also be written as x = 3k + 1. Substituting this into the second congruence and simplifying, we get » 9k ≡ −1 (mod 7)

Solving this congruence yields k ≡ 3 (mod 7), or k = 7l + 3. It then follows that x = 3 (7l + 3) + 1 = 21l + 10. Substituting this into the third congruence and simplifying, we get » 42l ≡ −16 (mod 8)

which has the solution l ≡ 0 (mod 4), or l = 4m. This yields x = 21(4m) + 10 = 84m + 10, or » x ≡ 10 (mod 84)

which describes all solutions to the system.

Further Information

Get more info on 'Linear Congruence Theorem'.


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