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Exercise 2.2Z: Galois Field GF(5)

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Addition/multiplication for  {a,b,c,d,e}

As in the  "Exercise 2.2"  we consider a finite field of order  q=5  and thus the Galois field

GF(5)={a,b,c,d,e}.

No further statements are made about the elements. They can be integers or any mathematical expressions.

The Galois field is determined exclusively by

  • an addition table modulo 5,
  • a multiplication table modulo 5.


The main properties of a Galois field are compiled on the  "first theory page".  There,  reference is made to

  • the commutative and distributive laws,
  • the neutral elements of addition and multiplication,
  • the inverse elements of addition and multiplication, and
  • the determination of primitive elements.


In the present example  β  would be a primitive element if  β2, β3  and  β4  (in general:  βq−1)  yield the remaining elements of the Galois field  GF(5)  except for the zero element.


Hint:  The exercise is related to the topic of the chapter  "Some Basics of Algebra".


Questions

1

Determine the neutral element of addition.

NA=a,
NA=b,
NA=c,
NA=d,
NA=e.

2

Determine the neutral element of multiplication.

NM=a,
NM=b,
NM=c,
NM=d,
NM=e.

3

If the commutative law is satisfied,

with respect to addition,  e.g.  a+b=b+a, ..., d+e=e+d,
with respect to multiplication,  e.g.  a⋅b=b⋅a, ..., d⋅e=e⋅d.

4

For which expressions is the distributive law satisfied?

a⋅(b+c)=a⋅b+a⋅c,
d⋅(b+c)=d⋅b+d⋅c,
e⋅(a+b)=e⋅a+e⋅b.

5

Replace  a, b, c, d, e  with elements of the number set  {0,1,2,3,4}  so that equal operation tables result.

a= 
b= 
c= 
d= 
e= 

6

What statements are true regarding inverse elements?

For all  zi∈{0,1,2,3,4}  there is an additive inverse.
Only for  zi∈{1,2,3,4}  there is an additive inverse.
For all  zi∈{0,1,2,3,4}  there is a multiplicative inverse.
Only for  zi∈{1,2,3,4}  there is a multiplicative inverse.

7

Which of the elements are primitive?

a=3.
b=2,
e=4.


Solution

Solution

(1)  The neutral element with respect to addition  (called  NA)  must satisfy the following equation for all elements  zi(i=0, ..., q−1):

zi+NA=NA+zi=zi.
  • From the addition table it follows  NA =d_.


(2)  In contrast,  for all elements  zi(i=1, ..., q−1),  the neutral element of multiplication  (NM)  satisfies the following condition:

zi⋅NM=NM⋅zi=zi.
  • From the multiplication table,  one can see  NM =c_.


(3)  The commutative law is satisfied for this Galois field in  both cases  (addition and multiplication), 
        since the addition table and multiplication table are each symmetric about the table diagonal.


(4)  Let us first consider the first expression.  If the distributive law is valid,  it must hold:

a⋅(b+c)=a⋅b+a⋅c.
  • For the left side you get:
a⋅(b+c)=a⋅a=e,
and for the right side:
a⋅b+a⋅c=c+a=e.
  • The distributive law is satisfied here as well as for the other two given expressions:
d⋅(b+c) = d⋅a=d,d⋅b+d⋅c=d+d=d,
e⋅(a+c) = e⋅e=c,e⋅a+e⋅c=b+e=c.

All proposed solutions  apply.


Transformed tables

(5)  The zero element   NA=d   becomes   NA=0 ⇒ d=0,  the identity element   NM=c   becomes NM=1 ⇒ c=1.

  • The other elements  a, b  and  e  can be determined modulo  5  from the addition table or the multiplication table.  For example,  from the first row of the addition table follows
(a+b)mod5=d=0.
  • Since both  a  and  b  cannot be  0  or  1  (since these are already assigned for  c  and  d  ),  the inference is:
a=2,b=3ora=3,b=2.
  • For example,  from the second row of the addition table it follows:
(b+b)mod5=e.
  • From  b=3  would result  e=1.  But this is again not possible,  because already  c=1  has been defined.
  • So you get as a final result:
a=3_,b=2_,c=1_,d=0_,e=4_.

The graph shows the addition and multiplication table for this set of numbers.


(6)  True are the   statements 1 and 4:

  • In the addition table,  one recognizes exactly one  d=0  in each row and column.  That is:
  • For all   zi∈{0,1,2,3,4}   there exists a unique additive inverse.
  • The multiplicative inverse can be recognized in the multiplication table by the entry  c=1.  The multiplicative inverses are as follows:
Row1:InvM(a=3) = b=2,
Row2:InvM(b=2) = a=3,
Row3:InvM(c=1) = c=1,
Row5:InvM(e=4) = e=4.
  • For the zero element  d=0  on the other hand,  no multiplicative inverse exists.


(7)  Concerning the primitive elements one obtains

a = 3,a2=9mod5=4,a3=27mod5=2,a4=81mod5=1⇒primitive,
b = 2,b2=4,b3=8mod5=3,b4=16mod5=1⇒primitive,
e = 4,e2=16mod5=1,e3=...=4,e4=...=1⇒notprimitive.
  • From the set  Z5={0,1,2,3,4}   ⇒   "2"  and  "3"  are primitive elements   ⇒   solution suggestions 1 and 2.

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