Difference between revisions of "Aufgaben:Exercise 2.2Z: Galois Field GF(5)"

From LNTwww
 
(21 intermediate revisions by 5 users not shown)
Line 1: Line 1:
−
{{quiz-Header|Buchseite=Kanalcodierung/Einige Grundlagen der Algebra}}
+
{{quiz-Header|Buchseite=Channel_Coding/Some_Basics_of_Algebra}}
  
−
[[File:P_ID2494__KC_Z_2_2.png|right|frame|Additions– und Multiplikationstabelle für $\{a, \, b, \, c, \, d, \, e\}$]]
+
[[File:P_ID2494__KC_Z_2_2.png|right|frame|Addition/multiplication for  $\rm \{a, \, b, \, c, \, d, \, e\}$]]
−
Wie in [[Aufgaben:2.2_Eigenschaften_von_Galoisfeldern|Aufgabe A2.2]] betrachten wir einen endlichen Körper der Ordnung $q = 5$ und damit das Galoisfeld
+
As in the  [[Aufgaben:Exercise_2.2:_Properties_of_Galois_Fields|"Exercise 2.2"]]  we consider a finite field of order  $q = 5$  and thus the Galois field
−
:$${\rm GF}(5) = \{{a}, { b},{c},{d},{e}\}\hspace{0.05cm}.$$
+
:$$\rm GF(5) = \{{a}, { b},{c},{d},{e}\}\hspace{0.05cm}.$$
  
−
Über die Elemente werden weiter keine Aussagen getroffen. Es können sowohl ganze Zahlen sein oder irgendwelche mathematische Ausdrücke. Das Galoisfeld wird ausschließlich bestimmt durch
+
No further statements are made about the elements. They can be integers or any mathematical expressions.
−
* eine Additionstabelle modulo 5,
 
−
* eine Multiplikationstabelle modulo 5,
 
  
 +
The Galois field is determined exclusively by
 +
* an addition table modulo 5,
  
−
Die wichtigsten Eigenschaften eines Galoisfeldes sind auf [[Kanalcodierung/Einige_Grundlagen_der_Algebra#Definition_eines_Galoisfeldes|Theorieseite 1]] zusammengestellt. In dieser Aufgabe wird Bezug genommen auf
+
* a multiplication table modulo 5.
−
* das Kommutativ– und das Distributivgesetz,
 
−
* die neutralen Elemente von Addition und Multiplikation,
 
−
* die inversen Elemente von Addition und Multiplikation, sowie
 
−
* die Bestimmung primitiver Elemente.
 
  
  
−
Im vorliegenden Beispiel wäre $\beta$ ein primitives Element, wenn $\beta^2, \ \beta^3$ und $\beta^4$ (allgemein: $\beta^{q-1})$ die übrigen Elemente des Galoisfeldes $\rm GF(5)$ mit Ausnahme des Nullelementes ergeben.
+
The main properties of a Galois field are compiled on the  [[Channel_Coding/Some_Basics_of_Algebra#Definition_of_a_Galois_field|"first theory page"]].  There,  reference is made to
 +
* the commutative and distributive laws,
  
−
''Hinweise:''
+
* the neutral elements of addition and multiplication,
−
* Die Aufgabe bezieht ich auf das Themengebiet des Kapitels [[Kanalcodierung/Einige_Grundlagen_der_Algebra| Einige Grundlagen der Algebra]].
+
−
* Sollte die Eingabe des Zahlenwertes „0” erforderlich sein, so geben Sie bitte „0.” ein.
+
* the inverse elements of addition and multiplication, and
  
 +
* the determination of primitive elements.
  
  
−
===Fragebogen===
+
In the present example  $\beta$  would be a primitive element if  $\beta^2, \ \beta^3$  and  $\beta^4$  $($in general:  $\beta^{q-1})$  yield the remaining elements of the Galois field  $\rm GF(5)$  except for the zero element.
 +
 
 +
 
 +
 
 +
Hint:  The exercise is related to the topic of the chapter  [[Channel_Coding/Some_Basics_of_Algebra| "Some Basics of Algebra"]].
 +
 
 +
 
 +
===Questions===
 
<quiz display=simple>
 
<quiz display=simple>
−
{Multiple-Choice
+
{Determine the neutral element of addition.
−
|type="[]"}
+
|type="()"}
−
+ correct
+
- $N_{\rm A} = \rm a$,
−
- false
+
- $N_{\rm A} = \rm b$,
 +
- $N_{\rm A} = \rm c$,
 +
+ $N_{\rm A} = \rm d$,
 +
- $N_{\rm A} = \rm e$.
  
−
{Multiple-Choice
+
{Determine the neutral element of multiplication.
−
|type="[]"}
+
|type="()"}
−
+ correct
+
- $N_{\rm M} = \rm a$,
−
- false
+
- $N_{\rm M} = \rm b$,
 +
+ $N_{\rm M} = \rm c$,
 +
- $N_{\rm M} = \rm d$,
 +
- $N_{\rm M} = \rm e$.
  
−
{Multiple-Choice
+
{If the commutative law is satisfied,
 
|type="[]"}
 
|type="[]"}
−
+ correct
+
+ with respect to addition,&nbsp; e.g.&nbsp; $\rm a + b = b + a, \hspace{0.05cm}\text{ ...} \hspace{0.1cm}, \ d + e = e + d$,
−
- false
+
+ with respect to multiplication,&nbsp; e.g.&nbsp; $\rm a \cdot b = b \cdot a, \hspace{0.05cm}\text{ ...} \hspace{0.1cm}, \ d \cdot e = e \cdot d$.
  
−
{Multiple-Choice
+
{For which expressions is the distributive law satisfied?
 
|type="[]"}
 
|type="[]"}
−
+ correct
+
+ $\rm a \cdot (b + c) = a \cdot b + a \cdot c$,
−
- false
+
+ $\rm d \cdot (b + c) = d \cdot b + d \cdot c$,
 +
+ $\rm e \cdot (a + b) = e \cdot a + e \cdot b$.
  
−
{Input-Box Frage
+
{Replace&nbsp; $\rm a, \ b, \ c, \ d, \ e$&nbsp; with elements of the number set&nbsp; $\{0, \, 1, \, 2, \, 3, \, 4\}$&nbsp; so that equal operation tables result.
 
|type="{}"}
 
|type="{}"}
−
$xyz \ = \ ${ 5.4 3% } $ab$
+
$\rm a \hspace{0.25cm} = \ ${ 3 }
 +
$\rm b \hspace{0.25cm}  = \ ${ 2 }
 +
$\rm c \hspace{0.25cm}  = \ ${ 1 }
 +
$\rm d \hspace{0.25cm}  = \ ${ 0. }
 +
$\rm e \hspace{0.25cm}  = \ ${ 4 }
  
−
{Multiple-Choice
+
{What statements are true regarding inverse elements?
 
|type="[]"}
 
|type="[]"}
−
+ correct
+
+ For all&nbsp; $z_i &#8712; \{0, \, 1, \, 2, \, 3, \, 4\}$&nbsp; there is an additive inverse.
−
- false
+
- Only for&nbsp; $z_i &#8712; \{1, \, 2, \, 3, \, 4\}$&nbsp; there is an additive inverse.
 +
- For all&nbsp; $z_i &#8712; \{0, \, 1, \, 2, \, 3, \, 4\}$&nbsp; there is a multiplicative inverse.
 +
+ Only for&nbsp; $z_i &#8712; \{1, \, 2, \, 3, \, 4\}$&nbsp; there is a multiplicative inverse.
  
−
{Multiple-Choice
+
{Which of the elements are primitive?
 
|type="[]"}
 
|type="[]"}
−
+ correct
+
+ $\rm a = 3$.
−
- false
+
+ $\rm b = 2$,
 +
- $\rm e = 4$.
 
</quiz>
 
</quiz>
  
−
===Musterlösung===
+
===Solution===
 
{{ML-Kopf}}
 
{{ML-Kopf}}
−
'''(1)'''&nbsp;  
+
'''(1)'''&nbsp; The neutral element with respect to addition&nbsp;  $($called&nbsp; $N_{\rm A})$&nbsp; must satisfy the following equation for all elements&nbsp; $z_i (i = 0, \hspace{0.05cm}\text{ ...} \hspace{0.1cm} , \ q-1)$:
 +
:$$z_i + N_{\rm A} = N_{\rm A} + z_i = z_i\hspace{0.05cm}.$$
  
 +
*From the addition table it follows&nbsp; $N_{\rm A} \ \underline{= \rm d}$.
  
−
'''(2)'''&nbsp;
 
  
  
−
'''(3)'''&nbsp;  
+
'''(2)'''&nbsp; In contrast,&nbsp; for all elements&nbsp; $z_i (i = 1,\hspace{0.05cm}\text{ ...} \hspace{0.1cm} , \ q-1)$,&nbsp; the neutral element of multiplication&nbsp; $(N_{\rm M})$&nbsp; satisfies the following condition:
 +
:$$z_i \cdot N_{\rm M} = N_{\rm M}\cdot z_i = z_i\hspace{0.05cm}.$$
  
 +
*From the multiplication table,&nbsp; one can see&nbsp; $N_{\rm M} \ \underline{= \rm c}$.
  
−
'''(4)'''&nbsp;
 
  
  
 +
'''(3)'''&nbsp; The commutative law is satisfied for this Galois field in&nbsp; <u>both cases</u>&nbsp; (addition and multiplication),&nbsp; <br> &nbsp; &nbsp; &nbsp; &nbsp; since the addition table and multiplication table are each symmetric about the table diagonal.
 +
 +
 +
 +
'''(4)'''&nbsp; Let us first consider the first expression.&nbsp; If the distributive law is valid,&nbsp; it must hold:
 +
:$$\rm a \cdot (b+c) = a \cdot b+ a \cdot c \hspace{0.05cm}.$$
 +
 +
*For the left side you get:
 +
:$$\rm a \cdot (b+c) = a \cdot a =e \hspace{0.05cm},$$
 +
 +
:and for the right side:
 +
:$$\rm a \cdot b+ a \cdot c =  c + a = e\hspace{0.05cm}.$$
 +
 +
*The distributive law is satisfied here as well as for the other two given expressions:
 +
:$$\rm d \cdot (b+c) \hspace{-0.1cm} \ = \ \hspace{-0.1cm} d \cdot a =d \hspace{0.05cm}, \hspace{0.5cm}d \cdot b+ d \cdot c =  d + d = d\hspace{0.05cm},$$
 +
:$$\rm e \cdot (a+c) \hspace{-0.1cm} \ = \ \hspace{-0.1cm} e \cdot e =c \hspace{0.05cm}, \hspace{0.5cm}e \cdot a+ e \cdot c =  b + e = c\hspace{0.05cm}.$$
 +
 +
<u>All proposed solutions</u>&nbsp; apply.
 +
 +
 +
 +
[[File:P_ID2495__KC_Z_2_2e.png|right|frame|Transformed tables]]
 
'''(5)'''&nbsp;  
 
'''(5)'''&nbsp;  
 +
The zero element &nbsp; $N_{\rm A} = \rm d$ &nbsp; becomes &nbsp; $N_{\rm A} = 0 \ \Rightarrow \ \rm d = 0$,&nbsp; the identity element &nbsp; $N_{\rm M} = \rm c$ &nbsp; becomes $N_{\rm M} = 1 \ \Rightarrow \ \rm c = 1$.
 +
 +
*The other elements&nbsp; $\rm a, \ b$&nbsp; and&nbsp; $\rm e$&nbsp; can be determined modulo&nbsp; $5$&nbsp; from the addition table or the multiplication table.&nbsp; For example,&nbsp; from the first row of the addition table follows
 +
:$$\rm (a + b) \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = d = 0 \hspace{0.05cm}.$$
 +
 +
*Since both&nbsp; $\rm a$&nbsp; and&nbsp; $\rm b$&nbsp; cannot be&nbsp; $0$&nbsp; or&nbsp; $1$&nbsp; (since these are already assigned for&nbsp; $\rm c$&nbsp; and&nbsp; $\rm d$&nbsp; ),&nbsp; the inference is:
 +
:$$\rm a = 2, \hspace{0.3cm} b = 3 \hspace{0.5cm}{\rm or}\hspace{0.5cm} a = 3, \hspace{0.3cm} b = 2\hspace{0.05cm}.$$
 +
 +
*For example,&nbsp; from the second row of the addition table it follows:
 +
:$$\rm (b + b) \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = e \hspace{0.05cm}.$$
 +
 +
*From&nbsp; $\rm b = 3$&nbsp; would result&nbsp; $\rm e = 1$.&nbsp; But this is again not possible,&nbsp; because already&nbsp; $\rm c = 1$&nbsp; has been defined.
 +
 +
*So you get as a final result:
 +
:$$\rm a \hspace{0.15cm}\underline{= 3}\hspace{0.05cm},\hspace{0.2cm}b \hspace{0.15cm}\underline{= 2}\hspace{0.05cm},\hspace{0.2cm}
 +
c \hspace{0.15cm}\underline{= 1}\hspace{0.05cm},\hspace{0.2cm}d \hspace{0.15cm}\underline{= 0}\hspace{0.05cm},\hspace{0.2cm}
 +
e \hspace{0.15cm}\underline{= 4}\hspace{0.05cm}.$$
 +
 +
The graph shows the addition and multiplication table for this set of numbers.
 +
 +
 +
 +
'''(6)'''&nbsp; True are the &nbsp; <u>statements 1 and 4</u>:
 +
*In the addition table,&nbsp; one recognizes exactly one&nbsp; $\rm d = 0$&nbsp; in each row and column.&nbsp; That is: 
 +
 +
*For all &nbsp; $z_i &#8712; \{0, \, 1, \, 2, \, 3, \, 4\}$ &nbsp; there exists a unique additive inverse.
 +
 +
*The multiplicative inverse can be recognized in the multiplication table by the entry&nbsp; $\rm c = 1$.&nbsp; The multiplicative inverses are as follows:
 +
:$${\rm Row \hspace{0.15cm}1\text{:}}\hspace{0.25cm} {\rm Inv_M}(a=3) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm b = 2 \hspace{0.05cm},$$
 +
:$${\rm Row\hspace{0.15cm} 2\text{:}}\hspace{0.25cm} {\rm Inv_M}(b=2) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm a=3 \hspace{0.05cm},$$
 +
:$${\rm Row\hspace{0.15cm} 3\text{:}}\hspace{0.25cm} {\rm Inv_M}(c=1) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm c=1 \hspace{0.05cm},$$
 +
:$${\rm Row\hspace{0.15cm} 5\text{:}}\hspace{0.25cm} {\rm Inv_M}(e=4) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm e=4 \hspace{0.05cm}.$$
 +
 +
*For the zero element&nbsp; $\rm d = 0$&nbsp; on the other hand,&nbsp;  no multiplicative inverse exists.
  
  
−
'''(6)'''&nbsp;
 
  
 +
'''(7)'''&nbsp; Concerning the primitive elements one obtains
 +
:$$\rm a \hspace{-0.15cm} \ = \ \hspace{-0.15cm} 3  \hspace{0.05cm},\hspace{0.2cm} a^2 = 9 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 4
 +
\hspace{0.05cm},\hspace{0.2cm} a^3 = 27 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 2\hspace{0.05cm},\hspace{0.2cm} a^4 = 81 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 1\hspace{0.13cm} \Rightarrow \hspace{0.13cm}{\rm primitive}\hspace{0.05cm},$$
 +
:$$\rm b \hspace{-0.15cm} \ = \ \hspace{-0.15cm} 2  \hspace{0.05cm},\hspace{0.2cm} b^2 = 4
 +
\hspace{0.05cm},\hspace{0.2cm} b^3 = 8 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 3\hspace{0.05cm},\hspace{0.2cm} b^4 = 16 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 1\hspace{0.13cm} \Rightarrow \hspace{0.13cm}{\rm primitive}\hspace{0.05cm},$$
 +
:$$\rm e \hspace{-0.15cm} \ = \ \hspace{-0.15cm} 4  \hspace{0.05cm},\hspace{0.2cm} e^2 = 16 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 1
 +
\hspace{0.05cm},\hspace{0.2cm} e^3 = \hspace{0.05cm} ...\hspace{0.05cm}= 4\hspace{0.05cm},\hspace{0.2cm} e^4 =\hspace{0.05cm} ...\hspace{0.05cm} = 1\hspace{0.13cm} \Rightarrow \hspace{0.13cm}{\rm not\hspace{0.15cm} primitive}\hspace{0.05cm}.$$
  
−
'''(7)'''&nbsp;  
+
*From the set&nbsp; $Z_5 = \{0, \, 1, \, 2, \, 3, \, 4\}$ &nbsp; &rArr; &nbsp;  "$2$"&nbsp; and&nbsp; "$3$"&nbsp; are primitive elements &nbsp; &#8658; &nbsp; <u>solution suggestions 1 and 2</u>.
 
{{ML-Fuß}}
 
{{ML-Fuß}}
  
  
  
−
[[Category:Aufgaben zu  Kanalcodierung|^2.1 Einige Grundlagen der Algebra^]]
+
[[Category:Channel Coding: Exercises|^2.1 Some Basics of Algebra^]]

Latest revision as of 16:04, 28 August 2022

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

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

$$\rm GF(5) = \{{a}, { b},{c},{d},{e}\}\hspace{0.05cm}.$$

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  $\beta$  would be a primitive element if  $\beta^2, \ \beta^3$  and  $\beta^4$  $($in general:  $\beta^{q-1})$  yield the remaining elements of the Galois field  $\rm GF(5)$  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.

$N_{\rm A} = \rm a$,
$N_{\rm A} = \rm b$,
$N_{\rm A} = \rm c$,
$N_{\rm A} = \rm d$,
$N_{\rm A} = \rm e$.

2

Determine the neutral element of multiplication.

$N_{\rm M} = \rm a$,
$N_{\rm M} = \rm b$,
$N_{\rm M} = \rm c$,
$N_{\rm M} = \rm d$,
$N_{\rm M} = \rm e$.

3

If the commutative law is satisfied,

with respect to addition,  e.g.  $\rm a + b = b + a, \hspace{0.05cm}\text{ ...} \hspace{0.1cm}, \ d + e = e + d$,
with respect to multiplication,  e.g.  $\rm a \cdot b = b \cdot a, \hspace{0.05cm}\text{ ...} \hspace{0.1cm}, \ d \cdot e = e \cdot d$.

4

For which expressions is the distributive law satisfied?

$\rm a \cdot (b + c) = a \cdot b + a \cdot c$,
$\rm d \cdot (b + c) = d \cdot b + d \cdot c$,
$\rm e \cdot (a + b) = e \cdot a + e \cdot b$.

5

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

$\rm a \hspace{0.25cm} = \ $
$\rm b \hspace{0.25cm} = \ $
$\rm c \hspace{0.25cm} = \ $
$\rm d \hspace{0.25cm} = \ $
$\rm e \hspace{0.25cm} = \ $

6

What statements are true regarding inverse elements?

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

7

Which of the elements are primitive?

$\rm a = 3$.
$\rm b = 2$,
$\rm e = 4$.


Solution

Solution

(1)  The neutral element with respect to addition  $($called  $N_{\rm A})$  must satisfy the following equation for all elements  $z_i (i = 0, \hspace{0.05cm}\text{ ...} \hspace{0.1cm} , \ q-1)$:

$$z_i + N_{\rm A} = N_{\rm A} + z_i = z_i\hspace{0.05cm}.$$
  • From the addition table it follows  $N_{\rm A} \ \underline{= \rm d}$.


(2)  In contrast,  for all elements  $z_i (i = 1,\hspace{0.05cm}\text{ ...} \hspace{0.1cm} , \ q-1)$,  the neutral element of multiplication  $(N_{\rm M})$  satisfies the following condition:

$$z_i \cdot N_{\rm M} = N_{\rm M}\cdot z_i = z_i\hspace{0.05cm}.$$
  • From the multiplication table,  one can see  $N_{\rm M} \ \underline{= \rm c}$.


(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:

$$\rm a \cdot (b+c) = a \cdot b+ a \cdot c \hspace{0.05cm}.$$
  • For the left side you get:
$$\rm a \cdot (b+c) = a \cdot a =e \hspace{0.05cm},$$
and for the right side:
$$\rm a \cdot b+ a \cdot c = c + a = e\hspace{0.05cm}.$$
  • The distributive law is satisfied here as well as for the other two given expressions:
$$\rm d \cdot (b+c) \hspace{-0.1cm} \ = \ \hspace{-0.1cm} d \cdot a =d \hspace{0.05cm}, \hspace{0.5cm}d \cdot b+ d \cdot c = d + d = d\hspace{0.05cm},$$
$$\rm e \cdot (a+c) \hspace{-0.1cm} \ = \ \hspace{-0.1cm} e \cdot e =c \hspace{0.05cm}, \hspace{0.5cm}e \cdot a+ e \cdot c = b + e = c\hspace{0.05cm}.$$

All proposed solutions  apply.


Transformed tables

(5)  The zero element   $N_{\rm A} = \rm d$   becomes   $N_{\rm A} = 0 \ \Rightarrow \ \rm d = 0$,  the identity element   $N_{\rm M} = \rm c$   becomes $N_{\rm M} = 1 \ \Rightarrow \ \rm c = 1$.

  • The other elements  $\rm a, \ b$  and  $\rm e$  can be determined modulo  $5$  from the addition table or the multiplication table.  For example,  from the first row of the addition table follows
$$\rm (a + b) \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = d = 0 \hspace{0.05cm}.$$
  • Since both  $\rm a$  and  $\rm b$  cannot be  $0$  or  $1$  (since these are already assigned for  $\rm c$  and  $\rm d$  ),  the inference is:
$$\rm a = 2, \hspace{0.3cm} b = 3 \hspace{0.5cm}{\rm or}\hspace{0.5cm} a = 3, \hspace{0.3cm} b = 2\hspace{0.05cm}.$$
  • For example,  from the second row of the addition table it follows:
$$\rm (b + b) \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = e \hspace{0.05cm}.$$
  • From  $\rm b = 3$  would result  $\rm e = 1$.  But this is again not possible,  because already  $\rm c = 1$  has been defined.
  • So you get as a final result:
$$\rm a \hspace{0.15cm}\underline{= 3}\hspace{0.05cm},\hspace{0.2cm}b \hspace{0.15cm}\underline{= 2}\hspace{0.05cm},\hspace{0.2cm} c \hspace{0.15cm}\underline{= 1}\hspace{0.05cm},\hspace{0.2cm}d \hspace{0.15cm}\underline{= 0}\hspace{0.05cm},\hspace{0.2cm} e \hspace{0.15cm}\underline{= 4}\hspace{0.05cm}.$$

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  $\rm d = 0$  in each row and column.  That is:
  • For all   $z_i ∈ \{0, \, 1, \, 2, \, 3, \, 4\}$   there exists a unique additive inverse.
  • The multiplicative inverse can be recognized in the multiplication table by the entry  $\rm c = 1$.  The multiplicative inverses are as follows:
$${\rm Row \hspace{0.15cm}1\text{:}}\hspace{0.25cm} {\rm Inv_M}(a=3) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm b = 2 \hspace{0.05cm},$$
$${\rm Row\hspace{0.15cm} 2\text{:}}\hspace{0.25cm} {\rm Inv_M}(b=2) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm a=3 \hspace{0.05cm},$$
$${\rm Row\hspace{0.15cm} 3\text{:}}\hspace{0.25cm} {\rm Inv_M}(c=1) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm c=1 \hspace{0.05cm},$$
$${\rm Row\hspace{0.15cm} 5\text{:}}\hspace{0.25cm} {\rm Inv_M}(e=4) \hspace{-0.15cm} \ = \ \hspace{-0.15cm} \rm e=4 \hspace{0.05cm}.$$
  • For the zero element  $\rm d = 0$  on the other hand,  no multiplicative inverse exists.


(7)  Concerning the primitive elements one obtains

$$\rm a \hspace{-0.15cm} \ = \ \hspace{-0.15cm} 3 \hspace{0.05cm},\hspace{0.2cm} a^2 = 9 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 4 \hspace{0.05cm},\hspace{0.2cm} a^3 = 27 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 2\hspace{0.05cm},\hspace{0.2cm} a^4 = 81 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 1\hspace{0.13cm} \Rightarrow \hspace{0.13cm}{\rm primitive}\hspace{0.05cm},$$
$$\rm b \hspace{-0.15cm} \ = \ \hspace{-0.15cm} 2 \hspace{0.05cm},\hspace{0.2cm} b^2 = 4 \hspace{0.05cm},\hspace{0.2cm} b^3 = 8 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 3\hspace{0.05cm},\hspace{0.2cm} b^4 = 16 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 1\hspace{0.13cm} \Rightarrow \hspace{0.13cm}{\rm primitive}\hspace{0.05cm},$$
$$\rm e \hspace{-0.15cm} \ = \ \hspace{-0.15cm} 4 \hspace{0.05cm},\hspace{0.2cm} e^2 = 16 \hspace{0.1cm}{\rm mod} \hspace{0.1cm} 5 = 1 \hspace{0.05cm},\hspace{0.2cm} e^3 = \hspace{0.05cm} ...\hspace{0.05cm}= 4\hspace{0.05cm},\hspace{0.2cm} e^4 =\hspace{0.05cm} ...\hspace{0.05cm} = 1\hspace{0.13cm} \Rightarrow \hspace{0.13cm}{\rm not\hspace{0.15cm} primitive}\hspace{0.05cm}.$$
  • From the set  $Z_5 = \{0, \, 1, \, 2, \, 3, \, 4\}$   ⇒   "$2$"  and  "$3$"  are primitive elements   ⇒   solution suggestions 1 and 2.

Retrieved from "http://en.lntwww.de/index.php?title=Aufgaben:Exercise_2.2Z:_Galois_Field_GF(5)&oldid=48627"
  • Powered by MediaWiki