Consider the field
𝑅
=
𝑍
2
[
𝑥
]
/
𝐼
where

𝐼
=
(
𝑥
4
+
𝑥
3
+
𝑥
2
+
𝑥
+
1
)
𝑍
2
[
𝑥
]
.

Find
𝛼
−
1
∈
𝑅
where

𝛼
=
𝐼
+
𝑥
+
1
.

Question

Consider the field 
𝑅
=
𝑍
2
[
𝑥
]
/
𝐼
 where

𝐼
=
(
𝑥
4
+
𝑥
3
+
𝑥
2
+
𝑥
+
1
)
𝑍
2
[
𝑥
]
.

Find 
𝛼
−
1
∈
𝑅
 where

𝛼
=
𝐼
+
𝑥
+
1
.

BlackTom AI · Accepted Answer

We start by clearly restating the given problem in our own words to ground our thinking.
- The ring in question is R = Z_2[x] / (x^4 + x^3 + x^2 + x + 1). Here Z_2 denotes the field with two elements, and we are taking polynomials modulo the ideal generated by f(x) = x^4 + x^3 + x^2 + x + 1.
- The element α is defined as α = I + x + 1, i.e., the coset of the polynomial x + 1 in the quotient.
- We are asked to find α − 1 ∈ R, which corresponds to the coset of (x + 1) − 1 = x in the quotient.
- Therefore, a direct computation in the quotient suggests α − 1 should correspond to the coset I + x (i.e., the class of x modulo f(x)).

Key steps and checks for the options (or lack thereof):
- Step 1: Recognize what α − 1 represents in the quotient. Since α = x + 1 modulo f(x), subtracting 1 leaves x modulo f(x). So α − 1 is the residue class of x in R, namely I + x.
- Step 2: Consider whether any simplification could rewrite x in terms of higher powers using the modulus f(x). In Z_2[x] modulo f(x), we only reduce by the relation f(x) ≡ 0. This gives x^4 ≡ x^3 + x^2 + x + 1 modulo f(x), but does not force x to equal e.g. x^3 + x or any other particular linear combination unless we actively perform a reduction of a specific higher-degree term to a lower-degree representative. Here, x already has degree 1, so there is no need to reduce it further.
- Step 3: Compare with any proposed expression. The given proposed answer (α − 1 = I + x^3 + x) would imply that x ≡ x^3 + x modulo f(x), i.e., x^3 ≡ 0 in R. But from the modulus f(x) we do not obtain x^3 ≡ 0 in general; we would have to show that x^3 is in the ideal generated by f(x), which it is not in this setup. Thus this proposed expression does not align with the standard reduction rules in R.

Why the lack of explicit answer options matters here:
- Since the provided data includes an answer field but no answer_options field, we cannot evaluate multiple-choice choices. The natural, direct deduction indicates that α − 1 corresponds to the coset represented by x, i.e., I + x. Any alternate expression would need to be validated by reducing to that same coset using the relation f(x) = 0 in R.
- The suggested alternative α − 1 = I + x^3 + x would imply a different residue class (x ≡ x^3 + x), which would force x^3 ≡ 0 in the quotient, a conclusion not supported by the modulus f(x).

In summary, based on standard quotient ring arithmetic, α − 1 is represented by the coset I + x (the class of x), and the proposed alternative I + x^3 + x does not match the straightforward reduction and appears incorrect unless supported by a specific reduction step that would justify x^3 ≡ 0 modulo f(x).

MATH3066 (ND) 2025 MATH3066 Exam Practice Multiple Choice Questions D

Consider the field 𝑅 = 𝑍 2 [ 𝑥 ] / 𝐼 where 𝐼 = ( 𝑥 4 + 𝑥 3 + 𝑥 2 + 𝑥 + 1 ) 𝑍 2 [ 𝑥 ] . Find 𝛼 − 1 ∈ 𝑅 where 𝛼 = 𝐼 + 𝑥 + 1 .

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