Which of the following statements are true and which are false? Give reasons for your answer.
(a) If a finite group GG acts on a finite set SS, then G_(s_(1))=G_(s_(2))G_{s_1}=G_{s_2} for all s_(1),s_(2)in Xs_1, s_2 \in X.
Answer:
The statement "If a finite group GG acts on a finite set SS, then G_(s_(1))=G_(s_(2))G_{s_1}=G_{s_2} for all s_(1),s_(2)in Ss_1, s_2 \in S" is false. Here, G_(s_(1))G_{s_1} and G_(s_(2))G_{s_2} denote the stabilizer subgroups of s_(1)s_1 and s_(2)s_2 in SS, respectively. The stabilizer subgroup G_(s)G_s of an element s in Ss \in S is defined as the set of all elements in GG that fix ss, i.e., G_(s)={g in G∣g*s=s}G_s = \{g \in G \mid g \cdot s = s\}.
Justification/Proof:
The statement claims that for any two elements s_(1)s_1 and s_(2)s_2 in the set SS, their stabilizer subgroups in GG are equal. This would mean that every element of GG that fixes s_(1)s_1 also fixes s_(2)s_2 and vice versa. However, this is not necessarily true for all actions of GG on SS. The equality of stabilizers for all elements in SS would imply a very rigid action, essentially saying that either the entire group fixes every element of SS or that the action is trivial (where GG acts on SS in such a way that every element of GG fixes every element of SS).
Counterexample:
Consider the group G={e,g}G = \{e, g\} where ee is the identity element and gg is an element of order 2 (meaning g^(2)=eg^2 = e), acting on the set S={s_(1),s_(2)}S = \{s_1, s_2\} with two elements. Define the action of GG on SS as follows:
ee fixes both s_(1)s_1 and s_(2)s_2 (as it must, being the identity).
gg swaps s_(1)s_1 and s_(2)s_2, i.e., g*s_(1)=s_(2)g \cdot s_1 = s_2 and g*s_(2)=s_(1)g \cdot s_2 = s_1.
For this action, we have:
G_(s_(1))={e}G_{s_1} = \{e\} because only the identity element fixes s_(1)s_1.
G_(s_(2))={e}G_{s_2} = \{e\} because only the identity element fixes s_(2)s_2.
In this specific example, G_(s_(1))G_{s_1} and G_(s_(2))G_{s_2} happen to be equal because the non-identity element of GG does not fix either of the elements of SS but rather swaps them. However, the action of gg clearly differentiates s_(1)s_1 and s_(2)s_2, showing that the nature of the action can lead to different behaviors for different elements of SS, and it’s easy to construct examples where G_(s_(1))!=G_(s_(2))G_{s_1} \neq G_{s_2} by adjusting the action.
For a more direct counterexample to the statement, consider a group GG acting on a set S={s_(1),s_(2),s_(3)}S = \{s_1, s_2, s_3\} where GG fixes s_(1)s_1 and s_(2)s_2 but not s_(3)s_3, or any scenario where the action is not uniform across all elements of SS. This shows that the original statement is false.
(b) There are exactly 8 elements of order 3 in S_(4)S_4.
Answer:
To determine the truth of the statement "There are exactly 8 elements of order 3 in S_(4)S_4," we need to understand the structure of the symmetric group S_(4)S_4 and the concept of an element’s order.
Background:
The symmetric group S_(4)S_4 consists of all permutations of four elements, with a total of 4!=244! = 24 elements.
The order of an element in a group is the smallest positive integer nn such that the element raised to the nnth power (under the group operation, which is composition in this case) equals the identity element. For permutations, this means repeating the permutation nn times returns the set to its original order.
Analysis:
Elements of order 3 in S_(4)S_4 are those permutations that cycle three elements while leaving the fourth unchanged. These can be represented as 3-cycles, such as (123)(123), which means that 1 goes to 2, 2 goes to 3, and 3 goes back to 1, with the fourth element (in this case, 4) remaining fixed.
To count the elements of order 3, we look for all possible 3-cycles within S_(4)S_4. A 3-cycle can be chosen by selecting 3 elements out of 4 to participate in the cycle, and the order in which these three elements are arranged matters (since (123)(123) is different from (132)(132), for example).
Counting 3-Cycles:
The number of ways to choose 3 elements out of 4 is given by the combination formula ((4)/(3))\binom{4}{3}. For each selection of 3 elements, there are 2 ways to arrange them into a cycle (since for any three distinct elements aa, bb, and cc, there are exactly two 3-cycles: (abc)(abc) and (acb)(acb)).
Thus, the total number of 3-cycles (elements of order 3) in S_(4)S_4 is:
After calculating, we find that there are indeed exactly 8 elements of order 3 in S_(4)S_4, which means the statement is true. Each of these elements is a permutation that cycles three of the four elements back to their original positions after three applications, which is the definition of having order 3.
(c) If F=Q(root(5)(2),root(3)(5))F=\mathbb{Q}(\sqrt[5]{2}, \sqrt[3]{5}), then [F:Q]=8[F: \mathbb{Q}]=8.
Answer:
To evaluate the statement "If F=Q(root(5)(2),root(3)(5))F=\mathbb{Q}(\sqrt[5]{2}, \sqrt[3]{5}), then [F:Q]=8[F: \mathbb{Q}]=8," we need to understand the concept of field extensions and the degree of an extension.
Background:
F=Q(root(5)(2),root(3)(5))F=\mathbb{Q}(\sqrt[5]{2}, \sqrt[3]{5}) is a field extension of Q\mathbb{Q} that includes both root(5)(2)\sqrt[5]{2} and root(3)(5)\sqrt[3]{5}.
The degree of the extension [F:Q][F: \mathbb{Q}] is the dimension of FF as a vector space over Q\mathbb{Q}.
The degree of a composite extension can be determined by the multiplicativity of degrees in tower extensions: if K sube L sube MK \subseteq L \subseteq M are fields, then [M:K]=[M:L][L:K][M:K] = [M:L][L:K].
Analysis:
Consider the extension Q(root(5)(2))\mathbb{Q}(\sqrt[5]{2}) over Q\mathbb{Q}. The minimal polynomial of root(5)(2)\sqrt[5]{2} over Q\mathbb{Q} is x^(5)-2x^5 – 2, which is irreducible over Q\mathbb{Q} by Eisenstein’s criterion (with prime p=2p=2). Thus, [Q(root(5)(2)):Q]=5[\mathbb{Q}(\sqrt[5]{2}):\mathbb{Q}] = 5.
Consider the extension Q(root(3)(5))\mathbb{Q}(\sqrt[3]{5}) over Q\mathbb{Q}. The minimal polynomial of root(3)(5)\sqrt[3]{5} over Q\mathbb{Q} is x^(3)-5x^3 – 5, which is also irreducible over Q\mathbb{Q} by Eisenstein’s criterion (with prime p=5p=5). Thus, [Q(root(3)(5)):Q]=3[\mathbb{Q}(\sqrt[3]{5}):\mathbb{Q}] = 3.
To find [F:Q][F: \mathbb{Q}] where F=Q(root(5)(2),root(3)(5))F=\mathbb{Q}(\sqrt[5]{2}, \sqrt[3]{5}), we need to consider how root(5)(2)\sqrt[5]{2} and root(3)(5)\sqrt[3]{5} interact within the extension. If these roots were to generate independent extensions (i.e., the extensions do not intersect in a way that reduces the overall degree), then we might expect the degree of FF over Q\mathbb{Q} to be the product of the degrees of the individual extensions. However, the actual degree of the combined extension depends on the interaction between these two extensions.
Given that the degrees of the individual extensions are 5 and 3, respectively, and assuming no prior knowledge about a direct interaction that would reduce the combined degree, one might naively expect the degree of the combined extension to be 5xx3=155 \times 3 = 15. This is because the extensions are generated by roots of polynomials that are irreducible over Q\mathbb{Q} and do not immediately suggest a common field that would reduce the degree of the extension.
Conclusion:
The statement "If F=Q(root(5)(2),root(3)(5))F=\mathbb{Q}(\sqrt[5]{2}, \sqrt[3]{5}), then [F:Q]=8[F: \mathbb{Q}]=8" is false. Based on the analysis above, without specific interaction that would reduce the degree, the expected degree of the field extension FF over Q\mathbb{Q} would naively be calculated as 1515, not 88, assuming independent extensions generated by root(5)(2)\sqrt[5]{2} and root(3)(5)\sqrt[3]{5}. However, the actual degree calculation requires detailed knowledge of how these extensions interact, and in general, the degree of such a combined extension is not simply the product of the degrees if there’s a nontrivial intersection. But in this specific case, without evidence of such an intersection that would lower the degree to 88, the initial analysis suggests the statement is incorrect.
To evaluate the statement "F_(7)(sqrt3)=F_(7)(sqrt5)\mathbf{F}_7(\sqrt{3})=\mathbf{F}_7(\sqrt{5})," we need to understand the structure of finite fields, specifically the field extensions of F_(7)\mathbf{F}_7, the finite field with 7 elements.
Background:
F_(7)\mathbf{F}_7 is the finite field with 7 elements, typically represented as {0,1,2,3,4,5,6}\{0, 1, 2, 3, 4, 5, 6\} with arithmetic operations performed modulo 7.
F_(7)(sqrt3)\mathbf{F}_7(\sqrt{3}) denotes the smallest field extension of F_(7)\mathbf{F}_7 containing sqrt3\sqrt{3}, and similarly, F_(7)(sqrt5)\mathbf{F}_7(\sqrt{5}) denotes the smallest field extension of F_(7)\mathbf{F}_7 containing sqrt5\sqrt{5}.
In the context of finite fields, an element aa is said to have a square root in F_(7)\mathbf{F}_7 if there exists an element b inF_(7)b \in \mathbf{F}_7 such that b^(2)-=amod7b^2 \equiv a \mod 7.
Analysis:
To determine whether F_(7)(sqrt3)=F_(7)(sqrt5)\mathbf{F}_7(\sqrt{3})=\mathbf{F}_7(\sqrt{5}), we need to check if sqrt3\sqrt{3} and sqrt5\sqrt{5} can be expressed in terms of each other within the field extensions of F_(7)\mathbf{F}_7.
Square Root of 3 in F_(7)\mathbf{F}_7: We need to find if there exists an element x inF_(7)x \in \mathbf{F}_7 such that x^(2)-=3mod7x^2 \equiv 3 \mod 7.
Square Root of 5 in F_(7)\mathbf{F}_7: Similarly, we need to find if there exists an element y inF_(7)y \in \mathbf{F}_7 such that y^(2)-=5mod7y^2 \equiv 5 \mod 7.
Let’s perform explicit checks for the existence of sqrt3\sqrt{3} and sqrt5\sqrt{5} in F_(7)\mathbf{F}_7:
For sqrt3\sqrt{3}: We look for an element xx such that x^(2)-=3mod7x^2 \equiv 3 \mod 7.
For sqrt5\sqrt{5}: We look for an element yy such that y^(2)-=5mod7y^2 \equiv 5 \mod 7.
Let’s calculate these explicitly to see if such elements exist and to understand the relationship between F_(7)(sqrt3)\mathbf{F}_7(\sqrt{3}) and F_(7)(sqrt5)\mathbf{F}_7(\sqrt{5}).
After calculating, we find that there are no integer solutions for x^(2)-=3mod7x^2 \equiv 3 \mod 7 and x^(2)-=5mod7x^2 \equiv 5 \mod 7 within F_(7)\mathbf{F}_7. This means that neither sqrt3\sqrt{3} nor sqrt5\sqrt{5} can be directly represented by any element in F_(7)\mathbf{F}_7 through simple squaring and modular arithmetic.
However, the statement "F_(7)(sqrt3)=F_(7)(sqrt5)\mathbf{F}_7(\sqrt{3})=\mathbf{F}_7(\sqrt{5})" refers to the equality of field extensions of F_(7)\mathbf{F}_7 that include sqrt3\sqrt{3} and sqrt5\sqrt{5}, respectively. Given that neither sqrt3\sqrt{3} nor sqrt5\sqrt{5} has a direct representation in F_(7)\mathbf{F}_7, both sqrt3\sqrt{3} and sqrt5\sqrt{5} would necessitate extensions of F_(7)\mathbf{F}_7 to be included.
The absence of integer solutions for these equations in F_(7)\mathbf{F}_7 suggests that both sqrt3\sqrt{3} and sqrt5\sqrt{5} would lead to the creation of quadratic extensions of F_(7)\mathbf{F}_7, as both would be roots of irreducible quadratic polynomials over F_(7)\mathbf{F}_7. The key insight here is not the absence of solutions within F_(7)\mathbf{F}_7 but the nature of the extensions they generate.
Given the structure of finite fields, any extension of F_(7)\mathbf{F}_7 to include a square root not in F_(7)\mathbf{F}_7 would have to be a quadratic extension, implying that both F_(7)(sqrt3)\mathbf{F}_7(\sqrt{3}) and F_(7)(sqrt5)\mathbf{F}_7(\sqrt{5}) are quadratic extensions of F_(7)\mathbf{F}_7. Since all quadratic extensions of a given finite field have the same size (in this case, 4949 elements, or F_(7^(2))\mathbf{F}_{7^2}), and since there’s only one field of each possible order in the realm of finite fields, it follows that F_(7)(sqrt3)\mathbf{F}_7(\sqrt{3}) and F_(7)(sqrt5)\mathbf{F}_7(\sqrt{5}) must indeed be the same field, despite the initial intuition suggested by the absence of direct square roots in F_(7)\mathbf{F}_7.
Therefore, the statement "F_(7)(sqrt3)=F_(7)(sqrt5)\mathbf{F}_7(\sqrt{3})=\mathbf{F}_7(\sqrt{5})" is true, as both sqrt3\sqrt{3} and sqrt5\sqrt{5} lead to the same quadratic extension of F_(7)\mathbf{F}_7, given the unique structure of finite fields and their extensions.
(e) For any alpha inF_(2^(5))^(**),alpha!=1,F_(2^(5))=F_(2)[alpha]\alpha \in \mathbb{F}_{2^5}^*, \alpha \neq 1, \mathbb{F}_{2^5}=\mathbb{F}_2[\alpha].
Answer:
The statement "For any alpha inF_(2^(5))^(**),alpha!=1,F_(2^(5))=F_(2)[alpha]\alpha \in \mathbb{F}_{2^5}^*, \alpha \neq 1, \mathbb{F}_{2^5}=\mathbb{F}_2[\alpha]" concerns the structure of finite fields, specifically the field with 2^(5)=322^5 = 32 elements, denoted F_(2^(5))\mathbb{F}_{2^5}, and the concept of field extensions.
Background:
F_(2^(5))\mathbb{F}_{2^5} is a finite field with 3232 elements. It is the splitting field for the polynomial x^(32)-xx^{32} – x over F_(2)\mathbb{F}_2, and it contains all roots of this polynomial.
F_(2^(5))^(**)\mathbb{F}_{2^5}^* denotes the multiplicative group of F_(2^(5))\mathbb{F}_{2^5}, which consists of all non-zero elements of F_(2^(5))\mathbb{F}_{2^5}. This group has 3131 elements, and it is cyclic.
F_(2)[alpha]\mathbb{F}_2[\alpha] denotes the smallest field containing both F_(2)\mathbb{F}_2 and the element alpha\alpha. If alpha\alpha is a primitive element of F_(2^(5))\mathbb{F}_{2^5} (i.e., a generator of the multiplicative group F_(2^(5))^(**)\mathbb{F}_{2^5}^*), then F_(2)[alpha]=F_(2^(5))\mathbb{F}_2[\alpha] = \mathbb{F}_{2^5}.
A primitive element of a finite field is an element whose powers generate all non-zero elements of the field.
Analysis:
The statement asserts that for any non-identity element alpha\alpha of F_(2^(5))^(**)\mathbb{F}_{2^5}^*, the field generated by alpha\alpha over F_(2)\mathbb{F}_2 is the entire field F_(2^(5))\mathbb{F}_{2^5}. This would mean that every non-identity element in F_(2^(5))\mathbb{F}_{2^5} is a primitive element.
However, this is not true for every non-identity element of F_(2^(5))\mathbb{F}_{2^5}. While it is true that a primitive element alpha\alpha (an element of order 3131) will generate the entire field F_(2^(5))\mathbb{F}_{2^5} through its powers, not every non-identity element in F_(2^(5))\mathbb{F}_{2^5} is a primitive element. Some elements will have smaller orders due to the structure of the cyclic group F_(2^(5))^(**)\mathbb{F}_{2^5}^*. For example, an element of order 33 would only cycle through 33 distinct non-zero elements (plus the identity element when raised to the power of 33) before repeating, and thus, it cannot generate the entire field.
Justification/Proof:
To disprove the statement, we can consider the property of the elements’ orders in F_(2^(5))^(**)\mathbb{F}_{2^5}^*. The order of any element alpha\alpha in a finite field must divide the order of the field’s multiplicative group, which is 3131 in this case (since F_(2^(5))^(**)\mathbb{F}_{2^5}^* has 3131 elements). By Lagrange’s theorem, the possible orders of elements in F_(2^(5))^(**)\mathbb{F}_{2^5}^* are divisors of 3131, which are 11 and 3131 itself, because 3131 is a prime number.
Therefore, while an element alpha\alpha with order 3131 (a primitive element) will indeed generate the entire field F_(2^(5))\mathbb{F}_{2^5} through its powers, implying F_(2^(5))=F_(2)[alpha]\mathbb{F}_{2^5} = \mathbb{F}_2[\alpha] for such alpha\alpha, not every non-identity element has this property, as the statement incorrectly generalizes. Only the primitive elements of F_(2^(5))\mathbb{F}_{2^5} can generate the entire field.
Conclusion:
The statement is false because not every non-identity element in F_(2^(5))\mathbb{F}_{2^5} is a primitive element capable of generating the entire field. Only specific elements, namely the primitive elements, have this property.