16.9 a) Show that an element a E(Z,,, O) is a unit iff (a,n) = 1.

Advanced Engineering Mathematics
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Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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16.9 a) Show that an element a E(Z,,, O) is a unit iff (a,n) = 1.
Transcribed Image Text:16.9 a) Show that an element a E(Z,,, O) is a unit iff (a,n) = 1.
Expert Solution
Step 1: Proving Condition 1

To show that an element a(Zn,,) is a unit if and only if (a,n)=1, need to prove two implications:

1. If a is a unit, then (a,n)=1.
2. If (a,n)=1, then a is a unit.

Let's proceeding with the proof:

1. If a is a unit, then (a,n)=1:

If a is a unit in the ring Zn, it means that there exists an element bZn such that ab=ba=1 (where 1 is the multiplicative identity in the ring).

By definition of the operation , will have ab=abmodn, which means that ab1modn. This implies that ab=1+kn for some integer k.

Now, let's applying the Euclidean algorithm to find the greatest common divisor of a and n:

d=gcd(a,n)

By definition, d must divide both a and n, so d divides ab. Therefore, d also divides 1+kn.

Since d divides n, it also divides kn. Therefore, d must divide 1.

The only positive integer that divides 1 is 1 itself. Therefore, d=1 and it has shown that (a,n)=1.


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