Calculations on $GF(16)$ find $0111/1111$
$begingroup$
It's my first time doing finite field arithmetics. As an exercise, I want to find $0111/1111 in GF(16)$ generated by $Pi(alpha)=1+alpha +alpha^4$ that is an irreducible polynomial.
In polynomial form we have:
- $0111 rightarrow alpha+alpha^2+alpha^3$
- $1111 rightarrow 1+alpha+alpha^2+alpha^3$
If I perform the polynomial division, I obtain $-1$ (that is the same result obtained writing $0111 equiv -1 pmod {1111}$).
How can I compute this result $-1$ in the right element of the field? Or perhaps this some kind of sign that the result $not in GF(16)$?
abstract-algebra finite-fields
$endgroup$
|
show 1 more comment
$begingroup$
It's my first time doing finite field arithmetics. As an exercise, I want to find $0111/1111 in GF(16)$ generated by $Pi(alpha)=1+alpha +alpha^4$ that is an irreducible polynomial.
In polynomial form we have:
- $0111 rightarrow alpha+alpha^2+alpha^3$
- $1111 rightarrow 1+alpha+alpha^2+alpha^3$
If I perform the polynomial division, I obtain $-1$ (that is the same result obtained writing $0111 equiv -1 pmod {1111}$).
How can I compute this result $-1$ in the right element of the field? Or perhaps this some kind of sign that the result $not in GF(16)$?
abstract-algebra finite-fields
$endgroup$
1
$begingroup$
You basically compute the inverse of 1111 in the field and then multiply 0111 with the inverse of 1111.
$endgroup$
– Wuestenfux
Dec 13 '18 at 11:35
$begingroup$
So I need to calculate $1111^-1$, the problem is that this process in $GF(16)$ is not clear
$endgroup$
– Alessar
Dec 13 '18 at 11:51
1
$begingroup$
The extended euclidean algorithm gives a very clear way of doing it.
$endgroup$
– ancientmathematician
Dec 13 '18 at 11:52
1
$begingroup$
There is only one $GF(16)$, in the sense that all fields of 16 elements are isomorphic to each other. But there are many ways to represent an element of $GF(16)$, which depend on the specific irreducible polynomial used. So your question as it stands is badly specified. It is true that you mention $Pi(alpha)$, but only as an afterthought; whereas without it, we can't know what you mean by $0111$ and $1111$.
$endgroup$
– TonyK
Dec 13 '18 at 12:34
2
$begingroup$
When I said "many", I should in fact have said "three". In addition to your $Pi(alpha)$, there is $1+alpha^3+alpha^4$ and $1+alpha+alpha^2+alpha^3+alpha^4$.
$endgroup$
– TonyK
Dec 13 '18 at 12:41
|
show 1 more comment
$begingroup$
It's my first time doing finite field arithmetics. As an exercise, I want to find $0111/1111 in GF(16)$ generated by $Pi(alpha)=1+alpha +alpha^4$ that is an irreducible polynomial.
In polynomial form we have:
- $0111 rightarrow alpha+alpha^2+alpha^3$
- $1111 rightarrow 1+alpha+alpha^2+alpha^3$
If I perform the polynomial division, I obtain $-1$ (that is the same result obtained writing $0111 equiv -1 pmod {1111}$).
How can I compute this result $-1$ in the right element of the field? Or perhaps this some kind of sign that the result $not in GF(16)$?
abstract-algebra finite-fields
$endgroup$
It's my first time doing finite field arithmetics. As an exercise, I want to find $0111/1111 in GF(16)$ generated by $Pi(alpha)=1+alpha +alpha^4$ that is an irreducible polynomial.
In polynomial form we have:
- $0111 rightarrow alpha+alpha^2+alpha^3$
- $1111 rightarrow 1+alpha+alpha^2+alpha^3$
If I perform the polynomial division, I obtain $-1$ (that is the same result obtained writing $0111 equiv -1 pmod {1111}$).
How can I compute this result $-1$ in the right element of the field? Or perhaps this some kind of sign that the result $not in GF(16)$?
abstract-algebra finite-fields
abstract-algebra finite-fields
edited Dec 13 '18 at 12:38
Alessar
asked Dec 13 '18 at 11:32
AlessarAlessar
308115
308115
1
$begingroup$
You basically compute the inverse of 1111 in the field and then multiply 0111 with the inverse of 1111.
$endgroup$
– Wuestenfux
Dec 13 '18 at 11:35
$begingroup$
So I need to calculate $1111^-1$, the problem is that this process in $GF(16)$ is not clear
$endgroup$
– Alessar
Dec 13 '18 at 11:51
1
$begingroup$
The extended euclidean algorithm gives a very clear way of doing it.
$endgroup$
– ancientmathematician
Dec 13 '18 at 11:52
1
$begingroup$
There is only one $GF(16)$, in the sense that all fields of 16 elements are isomorphic to each other. But there are many ways to represent an element of $GF(16)$, which depend on the specific irreducible polynomial used. So your question as it stands is badly specified. It is true that you mention $Pi(alpha)$, but only as an afterthought; whereas without it, we can't know what you mean by $0111$ and $1111$.
$endgroup$
– TonyK
Dec 13 '18 at 12:34
2
$begingroup$
When I said "many", I should in fact have said "three". In addition to your $Pi(alpha)$, there is $1+alpha^3+alpha^4$ and $1+alpha+alpha^2+alpha^3+alpha^4$.
$endgroup$
– TonyK
Dec 13 '18 at 12:41
|
show 1 more comment
1
$begingroup$
You basically compute the inverse of 1111 in the field and then multiply 0111 with the inverse of 1111.
$endgroup$
– Wuestenfux
Dec 13 '18 at 11:35
$begingroup$
So I need to calculate $1111^-1$, the problem is that this process in $GF(16)$ is not clear
$endgroup$
– Alessar
Dec 13 '18 at 11:51
1
$begingroup$
The extended euclidean algorithm gives a very clear way of doing it.
$endgroup$
– ancientmathematician
Dec 13 '18 at 11:52
1
$begingroup$
There is only one $GF(16)$, in the sense that all fields of 16 elements are isomorphic to each other. But there are many ways to represent an element of $GF(16)$, which depend on the specific irreducible polynomial used. So your question as it stands is badly specified. It is true that you mention $Pi(alpha)$, but only as an afterthought; whereas without it, we can't know what you mean by $0111$ and $1111$.
$endgroup$
– TonyK
Dec 13 '18 at 12:34
2
$begingroup$
When I said "many", I should in fact have said "three". In addition to your $Pi(alpha)$, there is $1+alpha^3+alpha^4$ and $1+alpha+alpha^2+alpha^3+alpha^4$.
$endgroup$
– TonyK
Dec 13 '18 at 12:41
1
1
$begingroup$
You basically compute the inverse of 1111 in the field and then multiply 0111 with the inverse of 1111.
$endgroup$
– Wuestenfux
Dec 13 '18 at 11:35
$begingroup$
You basically compute the inverse of 1111 in the field and then multiply 0111 with the inverse of 1111.
$endgroup$
– Wuestenfux
Dec 13 '18 at 11:35
$begingroup$
So I need to calculate $1111^-1$, the problem is that this process in $GF(16)$ is not clear
$endgroup$
– Alessar
Dec 13 '18 at 11:51
$begingroup$
So I need to calculate $1111^-1$, the problem is that this process in $GF(16)$ is not clear
$endgroup$
– Alessar
Dec 13 '18 at 11:51
1
1
$begingroup$
The extended euclidean algorithm gives a very clear way of doing it.
$endgroup$
– ancientmathematician
Dec 13 '18 at 11:52
$begingroup$
The extended euclidean algorithm gives a very clear way of doing it.
$endgroup$
– ancientmathematician
Dec 13 '18 at 11:52
1
1
$begingroup$
There is only one $GF(16)$, in the sense that all fields of 16 elements are isomorphic to each other. But there are many ways to represent an element of $GF(16)$, which depend on the specific irreducible polynomial used. So your question as it stands is badly specified. It is true that you mention $Pi(alpha)$, but only as an afterthought; whereas without it, we can't know what you mean by $0111$ and $1111$.
$endgroup$
– TonyK
Dec 13 '18 at 12:34
$begingroup$
There is only one $GF(16)$, in the sense that all fields of 16 elements are isomorphic to each other. But there are many ways to represent an element of $GF(16)$, which depend on the specific irreducible polynomial used. So your question as it stands is badly specified. It is true that you mention $Pi(alpha)$, but only as an afterthought; whereas without it, we can't know what you mean by $0111$ and $1111$.
$endgroup$
– TonyK
Dec 13 '18 at 12:34
2
2
$begingroup$
When I said "many", I should in fact have said "three". In addition to your $Pi(alpha)$, there is $1+alpha^3+alpha^4$ and $1+alpha+alpha^2+alpha^3+alpha^4$.
$endgroup$
– TonyK
Dec 13 '18 at 12:41
$begingroup$
When I said "many", I should in fact have said "three". In addition to your $Pi(alpha)$, there is $1+alpha^3+alpha^4$ and $1+alpha+alpha^2+alpha^3+alpha^4$.
$endgroup$
– TonyK
Dec 13 '18 at 12:41
|
show 1 more comment
2 Answers
2
active
oldest
votes
$begingroup$
As it happens, the discrete logarithm table for $GF(16)$ that I prepared for referrals like this, uses the same minimal polynomial of the primitive element. The only difference is that in the linked thread I denote by $gamma$ the element that you refer to as $alpha$.
Anyway, consulting that table, we see that
$$1111=1+alpha+alpha^2+alpha^3=alpha^{12},$$
and
$$
0111=alpha+alpha^2+alpha^3=alpha^{11}.
$$
Therefore
$$
begin{aligned}
frac{0111}{1111}&=frac{alpha^{11}}{alpha^{12}}=frac{alpha^{11}}{alpha^{12}}cdotfrac{alpha^3}{alpha^3}\
&=frac{alpha^{14}}{alpha^{15}}=frac{alpha^{14}}1\
&=alpha^{14}=alpha^3+1=1001.
end{aligned}
$$
$endgroup$
add a comment |
$begingroup$
$GF(16)$ has characteristic 2. That is, each coefficient of $alpha$ is either $0$ or $1$. And $-1=1$.
However, the polynomial division does not just result in $-1$ or $1$. Instead we have:
$$frac{alpha^3+alpha^2+alpha}{alpha^3+alpha^2+alpha+1}
= 1 + frac{1}{alpha^3+alpha^2+alpha+1}
$$
As an alternative to the answers in the comments, we can write each element (except $0$) as a power of $alpha$. After all, $GF(16)$ has a cyclic multiplicative group and $alpha^{15}=1$.
Over the polynomial $X^4+X+1$ we have $0111 = alpha^{11}$ and $1111=alpha^{12}$. Therefore:
$$0111/1111=alpha^{11}/alpha^{12}=alpha^{11+15-12}=alpha^{14}=1001$$
This is consistent with your result:
$$1 + frac{1}{alpha^3+alpha^2+alpha+1}=1+frac{1}{alpha^{12}}
=1+frac{alpha^{15}}{alpha^{12}}=1+alpha^3
$$
$endgroup$
$begingroup$
Good work without a log table! If prompted for an ad hoc way of finding the discrete log of $1+alpha+alpha^2+alpha^3$ I would do the following $$1+alpha+alpha^2+alpha^3=(1+alpha)^3=(alpha^4)^3=alpha^{12}.$$
$endgroup$
– Jyrki Lahtonen
Dec 13 '18 at 17:25
add a comment |
Your Answer
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
As it happens, the discrete logarithm table for $GF(16)$ that I prepared for referrals like this, uses the same minimal polynomial of the primitive element. The only difference is that in the linked thread I denote by $gamma$ the element that you refer to as $alpha$.
Anyway, consulting that table, we see that
$$1111=1+alpha+alpha^2+alpha^3=alpha^{12},$$
and
$$
0111=alpha+alpha^2+alpha^3=alpha^{11}.
$$
Therefore
$$
begin{aligned}
frac{0111}{1111}&=frac{alpha^{11}}{alpha^{12}}=frac{alpha^{11}}{alpha^{12}}cdotfrac{alpha^3}{alpha^3}\
&=frac{alpha^{14}}{alpha^{15}}=frac{alpha^{14}}1\
&=alpha^{14}=alpha^3+1=1001.
end{aligned}
$$
$endgroup$
add a comment |
$begingroup$
As it happens, the discrete logarithm table for $GF(16)$ that I prepared for referrals like this, uses the same minimal polynomial of the primitive element. The only difference is that in the linked thread I denote by $gamma$ the element that you refer to as $alpha$.
Anyway, consulting that table, we see that
$$1111=1+alpha+alpha^2+alpha^3=alpha^{12},$$
and
$$
0111=alpha+alpha^2+alpha^3=alpha^{11}.
$$
Therefore
$$
begin{aligned}
frac{0111}{1111}&=frac{alpha^{11}}{alpha^{12}}=frac{alpha^{11}}{alpha^{12}}cdotfrac{alpha^3}{alpha^3}\
&=frac{alpha^{14}}{alpha^{15}}=frac{alpha^{14}}1\
&=alpha^{14}=alpha^3+1=1001.
end{aligned}
$$
$endgroup$
add a comment |
$begingroup$
As it happens, the discrete logarithm table for $GF(16)$ that I prepared for referrals like this, uses the same minimal polynomial of the primitive element. The only difference is that in the linked thread I denote by $gamma$ the element that you refer to as $alpha$.
Anyway, consulting that table, we see that
$$1111=1+alpha+alpha^2+alpha^3=alpha^{12},$$
and
$$
0111=alpha+alpha^2+alpha^3=alpha^{11}.
$$
Therefore
$$
begin{aligned}
frac{0111}{1111}&=frac{alpha^{11}}{alpha^{12}}=frac{alpha^{11}}{alpha^{12}}cdotfrac{alpha^3}{alpha^3}\
&=frac{alpha^{14}}{alpha^{15}}=frac{alpha^{14}}1\
&=alpha^{14}=alpha^3+1=1001.
end{aligned}
$$
$endgroup$
As it happens, the discrete logarithm table for $GF(16)$ that I prepared for referrals like this, uses the same minimal polynomial of the primitive element. The only difference is that in the linked thread I denote by $gamma$ the element that you refer to as $alpha$.
Anyway, consulting that table, we see that
$$1111=1+alpha+alpha^2+alpha^3=alpha^{12},$$
and
$$
0111=alpha+alpha^2+alpha^3=alpha^{11}.
$$
Therefore
$$
begin{aligned}
frac{0111}{1111}&=frac{alpha^{11}}{alpha^{12}}=frac{alpha^{11}}{alpha^{12}}cdotfrac{alpha^3}{alpha^3}\
&=frac{alpha^{14}}{alpha^{15}}=frac{alpha^{14}}1\
&=alpha^{14}=alpha^3+1=1001.
end{aligned}
$$
answered Dec 13 '18 at 12:23
Jyrki LahtonenJyrki Lahtonen
109k13169372
109k13169372
add a comment |
add a comment |
$begingroup$
$GF(16)$ has characteristic 2. That is, each coefficient of $alpha$ is either $0$ or $1$. And $-1=1$.
However, the polynomial division does not just result in $-1$ or $1$. Instead we have:
$$frac{alpha^3+alpha^2+alpha}{alpha^3+alpha^2+alpha+1}
= 1 + frac{1}{alpha^3+alpha^2+alpha+1}
$$
As an alternative to the answers in the comments, we can write each element (except $0$) as a power of $alpha$. After all, $GF(16)$ has a cyclic multiplicative group and $alpha^{15}=1$.
Over the polynomial $X^4+X+1$ we have $0111 = alpha^{11}$ and $1111=alpha^{12}$. Therefore:
$$0111/1111=alpha^{11}/alpha^{12}=alpha^{11+15-12}=alpha^{14}=1001$$
This is consistent with your result:
$$1 + frac{1}{alpha^3+alpha^2+alpha+1}=1+frac{1}{alpha^{12}}
=1+frac{alpha^{15}}{alpha^{12}}=1+alpha^3
$$
$endgroup$
$begingroup$
Good work without a log table! If prompted for an ad hoc way of finding the discrete log of $1+alpha+alpha^2+alpha^3$ I would do the following $$1+alpha+alpha^2+alpha^3=(1+alpha)^3=(alpha^4)^3=alpha^{12}.$$
$endgroup$
– Jyrki Lahtonen
Dec 13 '18 at 17:25
add a comment |
$begingroup$
$GF(16)$ has characteristic 2. That is, each coefficient of $alpha$ is either $0$ or $1$. And $-1=1$.
However, the polynomial division does not just result in $-1$ or $1$. Instead we have:
$$frac{alpha^3+alpha^2+alpha}{alpha^3+alpha^2+alpha+1}
= 1 + frac{1}{alpha^3+alpha^2+alpha+1}
$$
As an alternative to the answers in the comments, we can write each element (except $0$) as a power of $alpha$. After all, $GF(16)$ has a cyclic multiplicative group and $alpha^{15}=1$.
Over the polynomial $X^4+X+1$ we have $0111 = alpha^{11}$ and $1111=alpha^{12}$. Therefore:
$$0111/1111=alpha^{11}/alpha^{12}=alpha^{11+15-12}=alpha^{14}=1001$$
This is consistent with your result:
$$1 + frac{1}{alpha^3+alpha^2+alpha+1}=1+frac{1}{alpha^{12}}
=1+frac{alpha^{15}}{alpha^{12}}=1+alpha^3
$$
$endgroup$
$begingroup$
Good work without a log table! If prompted for an ad hoc way of finding the discrete log of $1+alpha+alpha^2+alpha^3$ I would do the following $$1+alpha+alpha^2+alpha^3=(1+alpha)^3=(alpha^4)^3=alpha^{12}.$$
$endgroup$
– Jyrki Lahtonen
Dec 13 '18 at 17:25
add a comment |
$begingroup$
$GF(16)$ has characteristic 2. That is, each coefficient of $alpha$ is either $0$ or $1$. And $-1=1$.
However, the polynomial division does not just result in $-1$ or $1$. Instead we have:
$$frac{alpha^3+alpha^2+alpha}{alpha^3+alpha^2+alpha+1}
= 1 + frac{1}{alpha^3+alpha^2+alpha+1}
$$
As an alternative to the answers in the comments, we can write each element (except $0$) as a power of $alpha$. After all, $GF(16)$ has a cyclic multiplicative group and $alpha^{15}=1$.
Over the polynomial $X^4+X+1$ we have $0111 = alpha^{11}$ and $1111=alpha^{12}$. Therefore:
$$0111/1111=alpha^{11}/alpha^{12}=alpha^{11+15-12}=alpha^{14}=1001$$
This is consistent with your result:
$$1 + frac{1}{alpha^3+alpha^2+alpha+1}=1+frac{1}{alpha^{12}}
=1+frac{alpha^{15}}{alpha^{12}}=1+alpha^3
$$
$endgroup$
$GF(16)$ has characteristic 2. That is, each coefficient of $alpha$ is either $0$ or $1$. And $-1=1$.
However, the polynomial division does not just result in $-1$ or $1$. Instead we have:
$$frac{alpha^3+alpha^2+alpha}{alpha^3+alpha^2+alpha+1}
= 1 + frac{1}{alpha^3+alpha^2+alpha+1}
$$
As an alternative to the answers in the comments, we can write each element (except $0$) as a power of $alpha$. After all, $GF(16)$ has a cyclic multiplicative group and $alpha^{15}=1$.
Over the polynomial $X^4+X+1$ we have $0111 = alpha^{11}$ and $1111=alpha^{12}$. Therefore:
$$0111/1111=alpha^{11}/alpha^{12}=alpha^{11+15-12}=alpha^{14}=1001$$
This is consistent with your result:
$$1 + frac{1}{alpha^3+alpha^2+alpha+1}=1+frac{1}{alpha^{12}}
=1+frac{alpha^{15}}{alpha^{12}}=1+alpha^3
$$
answered Dec 13 '18 at 12:25
I like SerenaI like Serena
4,1771722
4,1771722
$begingroup$
Good work without a log table! If prompted for an ad hoc way of finding the discrete log of $1+alpha+alpha^2+alpha^3$ I would do the following $$1+alpha+alpha^2+alpha^3=(1+alpha)^3=(alpha^4)^3=alpha^{12}.$$
$endgroup$
– Jyrki Lahtonen
Dec 13 '18 at 17:25
add a comment |
$begingroup$
Good work without a log table! If prompted for an ad hoc way of finding the discrete log of $1+alpha+alpha^2+alpha^3$ I would do the following $$1+alpha+alpha^2+alpha^3=(1+alpha)^3=(alpha^4)^3=alpha^{12}.$$
$endgroup$
– Jyrki Lahtonen
Dec 13 '18 at 17:25
$begingroup$
Good work without a log table! If prompted for an ad hoc way of finding the discrete log of $1+alpha+alpha^2+alpha^3$ I would do the following $$1+alpha+alpha^2+alpha^3=(1+alpha)^3=(alpha^4)^3=alpha^{12}.$$
$endgroup$
– Jyrki Lahtonen
Dec 13 '18 at 17:25
$begingroup$
Good work without a log table! If prompted for an ad hoc way of finding the discrete log of $1+alpha+alpha^2+alpha^3$ I would do the following $$1+alpha+alpha^2+alpha^3=(1+alpha)^3=(alpha^4)^3=alpha^{12}.$$
$endgroup$
– Jyrki Lahtonen
Dec 13 '18 at 17:25
add a comment |
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1
$begingroup$
You basically compute the inverse of 1111 in the field and then multiply 0111 with the inverse of 1111.
$endgroup$
– Wuestenfux
Dec 13 '18 at 11:35
$begingroup$
So I need to calculate $1111^-1$, the problem is that this process in $GF(16)$ is not clear
$endgroup$
– Alessar
Dec 13 '18 at 11:51
1
$begingroup$
The extended euclidean algorithm gives a very clear way of doing it.
$endgroup$
– ancientmathematician
Dec 13 '18 at 11:52
1
$begingroup$
There is only one $GF(16)$, in the sense that all fields of 16 elements are isomorphic to each other. But there are many ways to represent an element of $GF(16)$, which depend on the specific irreducible polynomial used. So your question as it stands is badly specified. It is true that you mention $Pi(alpha)$, but only as an afterthought; whereas without it, we can't know what you mean by $0111$ and $1111$.
$endgroup$
– TonyK
Dec 13 '18 at 12:34
2
$begingroup$
When I said "many", I should in fact have said "three". In addition to your $Pi(alpha)$, there is $1+alpha^3+alpha^4$ and $1+alpha+alpha^2+alpha^3+alpha^4$.
$endgroup$
– TonyK
Dec 13 '18 at 12:41