Let p be a prime such that p mod 4 = 3. How many elements oforder 4 are in GF(pn)*?

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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Let p be a prime such that p mod 4 = 3. How many elements of
order 4 are in GF(pn)*?

Expert Solution
Step 1

Given: A group G= GF(pn) and the relation is p3 mod4.

To Find: Number of elements of order 4.

Step 2

Let G be a group GF(pn).

the given relation is p3 mod4....................(i)

We know that GF(pn) is a multiplicative group which is isomorphic to pn-1.

pn-1 has an element of order 4 if 4|pn-1.

this implies there exists k, such that,

pn-1=4k...................(ii)

from (i),

pn3n mod4.

Now from (ii),

4k+13n mod44k(3n-1) mod4.

Now since 4k is always divisible by 4,

3n-1=0,4,8,12,....

thus the possibilities of are,

n=2,4,6,8,...

 n is even.

Hence both relations (i) and (ii) are possible when n is even.

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