MMT-003 Solved Assignment 2023

IGNOU MMT-003 Solved Assignment 2023 | M.Sc. MACS

Solved By – Narendra Kr. Sharma – M.Sc (Mathematics Honors) – Delhi University

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IGNOU MMT-003 Assignment Question Paper 2023

mmt-003-2023-ext-aa2d2d76-613f-4e79-9c79-c97616952506
  1. Which of the following statements are true? Give reasons for your answers. Marks will only be given for valid justification of your answers.
i) If G G GGG is a finite abelian group and p p ppp is a prime factor of o ( G ) o ( G ) o(G)o(G)o(G), then the number of Sylow p-subgroups of G G G\mathrm{G}G is a prime.
ii) The minimum polynomial of 5 1 / 3 5 1 / 3 5^(1//3)5^{1 / 3}51/3 over Q Q Q\mathbb{Q}Q is x 1 / 3 x 1 / 3 x^(1//3)x^{1 / 3}x1/3.
iii) Z m n Z m × Z h m , n Z Z m n Z m × Z h m , n Z Z_(mn)≃Z_(m)xxZ_(h)AAm,ninZ\mathbb{Z}_{\mathrm{mn}} \simeq \mathbb{Z}_{\mathrm{m}} \times \mathbb{Z}_{\mathrm{h}} \forall \mathrm{m}, \mathrm{n} \in \mathbb{Z}ZmnZm×Zhm,nZ
iv) If G G GGG is a finite group and m o ( G ) , m N m o ( G ) , m N m∣o(G),m inNm \mid o(G), m \in \mathbb{N}mo(G),mN, then G G GGG has a subgroup of order m m mmm.
v) If β 1 β 1 beta_(1)\beta_{1}β1 and β 2 β 2 beta_(2)\beta_{2}β2 are two 15 th 15 th  15^(“th “)15^{\text {th }}15th  roots of unity, then Q ( β 1 ) = Q ( β 2 ) Q β 1 = Q β 2 Q(beta_(1))=Q(beta_(2))\mathbb{Q}\left(\beta_{1}\right)=\mathbb{Q}\left(\beta_{2}\right)Q(β1)=Q(β2).
vi) There exists an extension field of Z 3 Z 3 Z_(3)\mathbb{Z}_{3}Z3 of order 25 .
vii) Every group of order 18 has a normal subgroup of order 2.
viii) If I I I\mathrm{I}I and J J J\mathrm{J}J are ideals of a ring R R R\mathrm{R}R, then I J = I J I J = I J IJ=InnJ\mathrm{IJ}=\mathrm{I} \cap \mathrm{J}IJ=IJ.
ix) If f : R S f : R S f:RrarrS\mathrm{f}: \mathrm{R} \rightarrow \mathrm{S}f:RS is a ring homomorphism and I I I\mathrm{I}I is an ideal of R R R\mathrm{R}R, then f ( I ) f ( I ) f(I)\mathrm{f}(\mathrm{I})f(I) is an ideal of S.
x x x\mathrm{x}x Every prime ideal of an integral domain is a maximal ideal.
  1. a) Let G G GGG be a group and let H G , K G , o ( H ) = o ( K ) = p H G , K G , o ( H ) = o ( K ) = p H <= G,K <= G,o(H)=o(K)=pH \leq G, K \leq G, o(H)=o(K)=pHG,KG,o(H)=o(K)=p, a prime. Show that either H K = { e } H K = { e } HnnK={e}\mathrm{H} \cap \mathrm{K}=\{\mathrm{e}\}HK={e} or H = K H = K H=K\mathrm{H}=\mathrm{K}H=K. Is this result still true if p p p\mathrm{p}p is not a prime? Justify your answer.
b) Let G G G\mathrm{G}G be the group of all rigid motions of a plane and S S S\mathrm{S}S be the set of all rectangles in the plane. Show that G G GGG acts on S . Also obtain the orbit and stabiliser of a square under this action.
c) Let G G GGG be a finite group and H H HHH be a normal subgroup of G G GGG. Prove that H = C x H = C x H=uuC_(x)H=\cup C_{x}H=Cx where the C x C x C_(x)C_{x}Cx are all the distinct conjugacy classes of G G GGG such that H C x H C x H nnC_(x)!=O/H \cap C_{x} \neq \varnothingHCx.
  1. a) Find the number of Sylow 5-subgroups, Sylow 7-subgroups and Sylow 2-subgroups A 5 A 5 A_(5)A_{5}A5 has.
b) Let G 1 G 1 G_(1)G_{1}G1 and G 2 G 2 G_(2)G_{2}G2 be finite groups such that p p ppp divides | G 1 | G 1 |G_(1)|\left|G_{1}\right||G1| and | G 2 | G 2 |G_(2)|\left|G_{2}\right||G2|. Prove that the Sylow p-subgroups of G 1 × G 2 G 1 × G 2 G_(1)xxG_(2)G_{1} \times G_{2}G1×G2 are precisely of the form P 1 × P 2 P 1 × P 2 P_(1)xxP_(2)P_{1} \times P_{2}P1×P2, where P 1 P 1 P_(1)P_{1}P1 and P 2 P 2 P_(2)P_{2}P2 are Sylow p-subgroups of G 1 G 1 G_(1)G_{1}G1 and G 2 G 2 G_(2)G_{2}G2, respectively.
  1. a) Write [ 1 1 0 2 0 3 5 5 1 ] 1 1 0 2 0 3 5 5 1 [[1,-1,0],[2,0,3],[5,5,1]]\left[\begin{array}{ccc}1 & -1 & 0 \\ 2 & 0 & 3 \\ 5 & 5 & 1\end{array}\right][110203551] as a product of O ( 3 ) O ( 3 ) O(3)\mathrm{O}(3)O(3) and an element of B 3 ( R ) B 3 ( R ) B_(3)(R)B_{3}(\mathbb{R})B3(R).
b) Show that S U ( 2 ) S U ( 2 ) SU(2)\mathrm{SU}(2)SU(2) and S 3 S 3 S^(3)\mathrm{S}^{3}S3 are structurally the same.
  1. a) Find all the possible abelian groups, up to isomorphism, of order 900 .
b) Construct the free group on the set { α , β , γ } { α , β , γ } {alpha,beta,gamma}\{\alpha, \beta, \gamma\}{α,β,γ}. Further, check if it is a free abelian group or not.
  1. a) Check whether or not the ring R = Z 3 [ x ] / < x 6 1 > R = Z 3 [ x ] / < x 6 1 > R=Z_(3)[x]// < x^(6)-1 >\mathrm{R}=\mathbb{Z}_{3}[\mathrm{x}] /<\mathrm{x}^{6}-1>R=Z3[x]/<x61>
    i) is finite;
    ii) has zero divisors;
    iii) has nilpotent elements.
b) Give two distinct rings whose quotient field is { a + i b a , b R } { a + i b a , b R } {a+ib∣a,b inR}\{a+i b \mid a, b \in \mathbb{R}\}{a+iba,bR}. Justify your answer.
c) Evaluate the following Legendre Symbols:
i) ( 139 431 ) 139 431 ((139)/(431))\left(\frac{139}{431}\right)(139431)
ii) ( 149 439 ) 149 439 ((149)/(439))\left(\frac{149}{439}\right)(149439)
d) Check whether 9782957210008 is a valid ISBN number.
  1. a) Check whether or not Z [ 7 ] Z [ 7 ] Z[sqrt(-7)]\mathbb{Z}[\sqrt{-7}]Z[7] is a Euclidean domain.
b) Use the division algorithm to find the inverse of 18 ¯ 18 ¯ bar(18)\overline{18}18¯ in Z 35 Z 35 Z_(35)\mathbb{Z}_{35}Z35.
  1. i) Let G G GGG be a group of automorphisms of a field K K KKK. Is the fixed field K G K G K^(G)K^{G}KG a subfield of K K K\mathrm{K}K ? Why, or why not?
ii) Find K G K G K^(G)K^{G}KG, where K = Q ( i , 3 ) , G = G ( K / Q ) K = Q ( i , 3 ) , G = G ( K / Q ) K=Q(i,sqrt3),G=G(K//Q)K=\mathbb{Q}(i, \sqrt{3}), G=G(K / \mathbb{Q})K=Q(i,3),G=G(K/Q).
\(cos\:2\theta =2\:cos^2\theta -1\)

MMT-003 Sample Solution 2023

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MMT-003 Solved Assignment 2023

Question:-01

  1. Which of the following statements are true? Give reasons for your answers. Marks will only be given for valid justification of your answers.
    i) If G G G\mathrm{G}G is a finite abelian group and p p p\mathrm{p}p is a prime factor of o ( G ) o ( G ) o(G)\mathrm{o}(\mathrm{G})o(G), then the number of Sylow p-subgroups of G G G\mathrm{G}G is a prime.
Answer:
The statement “If G G GGG is a finite abelian group and p p ppp is a prime factor of o ( G ) o ( G ) o(G)o(G)o(G), then the number of Sylow p p ppp-subgroups of G G GGG is a prime” is false.

Justification:

In a finite abelian group G G GGG, every Sylow p p ppp-subgroup is normal. Therefore, all Sylow p p ppp-subgroups are conjugate to each other, and they are all isomorphic. In an abelian group, the number of Sylow p p ppp-subgroups is given by n p n p n_(p)n_pnp, and according to Sylow’s Third Theorem, n p n p n_(p)n_pnp divides | G | | G | |G||G||G| and n p 1 mod p n p 1 mod p n_(p)-=1modpn_p \equiv 1 \mod pnp1modp.
However, there is no requirement that n p n p n_(p)n_pnp must be a prime number.

Counterexample:

Consider the abelian group G = Z 4 × Z 2 G = Z 4 × Z 2 G=Z_(4)xxZ_(2)G = \mathbb{Z}_4 \times \mathbb{Z}_2G=Z4×Z2, where Z n Z n Z_(n)\mathbb{Z}_nZn denotes the integers modulo n n nnn. The order of G G GGG is o ( G ) = 4 × 2 = 8 o ( G ) = 4 × 2 = 8 o(G)=4xx2=8o(G) = 4 \times 2 = 8o(G)=4×2=8.
The prime factors of o ( G ) o ( G ) o(G)o(G)o(G) are 2 2 222 and 2 2 222 (i.e., 8 = 2 3 8 = 2 3 8=2^(3)8 = 2^38=23).
The Sylow 2 2 222-subgroups of G G GGG are generated by ( 2 , 0 ) ( 2 , 0 ) (2,0)(2, 0)(2,0), ( 0 , 1 ) ( 0 , 1 ) (0,1)(0, 1)(0,1), and ( 2 , 1 ) ( 2 , 1 ) (2,1)(2, 1)(2,1). So, there are 3 3 333 Sylow 2 2 222-subgroups, and 3 3 333 is not a prime divisor of 8 8 888.
Thus, the statement is false, as demonstrated by this counterexample.

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ii) The minimum polynomial of 5 1 / 3 5 1 / 3 5^(1//3)5^{1 / 3}51/3 over Q Q Q\mathbb{Q}Q is x 1 / 3 x 1 / 3 x^(1//3)x^{1 / 3}x1/3.
Answer:
The statement “The minimum polynomial of 5 1 / 3 5 1 / 3 5^(1//3)5^{1/3}51/3 over Q Q Q\mathbb{Q}Q is x 1 / 3 x 1 / 3 x^(1//3)x^{1/3}x1/3” is false.

Justification:

The minimum polynomial of an algebraic number α α alpha\alphaα over a field F F FFF is the unique monic polynomial f ( x ) f ( x ) f(x)f(x)f(x) of least degree such that f ( α ) = 0 f ( α ) = 0 f(alpha)=0f(\alpha) = 0f(α)=0 and f ( x ) f ( x ) f(x)f(x)f(x) has coefficients in F F FFF.
For 5 1 / 3 5 1 / 3 5^(1//3)5^{1/3}51/3, the minimum polynomial over Q Q Q\mathbb{Q}Q is x 3 5 x 3 5 x^(3)-5x^3 – 5x35, not x 1 / 3 x 1 / 3 x^(1//3)x^{1/3}x1/3.
  1. x 3 5 x 3 5 x^(3)-5x^3 – 5x35 is a polynomial with coefficients in Q Q Q\mathbb{Q}Q.
  2. ( 5 1 / 3 ) 3 5 = 5 5 = 0 ( 5 1 / 3 ) 3 5 = 5 5 = 0 (5^(1//3))^(3)-5=5-5=0(5^{1/3})^3 – 5 = 5 – 5 = 0(51/3)35=55=0, so 5 1 / 3 5 1 / 3 5^(1//3)5^{1/3}51/3 is a root of x 3 5 x 3 5 x^(3)-5x^3 – 5x35.
  3. x 3 5 x 3 5 x^(3)-5x^3 – 5x35 is irreducible over Q Q Q\mathbb{Q}Q, which means it has the least degree among all polynomials that have 5 1 / 3 5 1 / 3 5^(1//3)5^{1/3}51/3 as a root and have coefficients in Q Q Q\mathbb{Q}Q.
Moreover, x 1 / 3 x 1 / 3 x^(1//3)x^{1/3}x1/3 is not even a polynomial over Q Q Q\mathbb{Q}Q because its exponent 1 / 3 1 / 3 1//31/31/3 is not a non-negative integer.
Thus, the statement is false.

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iii) Z m n Z m × Z n m , n Z Z m n Z m × Z n m , n Z Z_(mn)≃Z_(m)xxZ_(n)AAm,ninZ\mathbb{Z}_{\mathrm{mn}} \simeq \mathbb{Z}_{\mathrm{m}} \times \mathbb{Z}_{\mathrm{n}} \forall \mathrm{m}, \mathrm{n} \in \mathbb{Z}ZmnZm×Znm,nZ
Answer:
The statement “ Z m n Z m × Z n m , n Z Z m n Z m × Z n m , n Z Z_(mn)≃Z_(m)xxZ_(n)AA m,n inZ\mathbb{Z}_{mn} \simeq \mathbb{Z}_{m} \times \mathbb{Z}_{n} \forall m, n \in \mathbb{Z}ZmnZm×Znm,nZ” is false.

Justification:

The isomorphism Z m n Z m × Z n Z m n Z m × Z n Z_(mn)≃Z_(m)xxZ_(n)\mathbb{Z}_{mn} \simeq \mathbb{Z}_{m} \times \mathbb{Z}_{n}ZmnZm×Zn holds if and only if m m mmm and n n nnn are coprime (i.e., gcd ( m , n ) = 1 gcd ( m , n ) = 1 gcd(m,n)=1\gcd(m, n) = 1gcd(m,n)=1).

Counterexample:

Let m = 4 m = 4 m=4m = 4m=4 and n = 2 n = 2 n=2n = 2n=2. Then Z 4 × 2 = Z 8 Z 4 × 2 = Z 8 Z_(4xx2)=Z_(8)\mathbb{Z}_{4 \times 2} = \mathbb{Z}_8Z4×2=Z8.
However, Z 4 × Z 2 Z 4 × Z 2 Z_(4)xxZ_(2)\mathbb{Z}_4 \times \mathbb{Z}_2Z4×Z2 is not isomorphic to Z 8 Z 8 Z_(8)\mathbb{Z}_8Z8.
  1. Z 8 Z 8 Z_(8)\mathbb{Z}_8Z8 has elements [ 0 ] , [ 1 ] , [ 2 ] , [ 3 ] , [ 4 ] , [ 5 ] , [ 6 ] , [ 7 ] [ 0 ] , [ 1 ] , [ 2 ] , [ 3 ] , [ 4 ] , [ 5 ] , [ 6 ] , [ 7 ] [0],[1],[2],[3],[4],[5],[6],[7][0], [1], [2], [3], [4], [5], [6], [7][0],[1],[2],[3],[4],[5],[6],[7] and is cyclic, generated by [ 1 ] [ 1 ] [1][1][1].
  2. Z 4 × Z 2 Z 4 × Z 2 Z_(4)xxZ_(2)\mathbb{Z}_4 \times \mathbb{Z}_2Z4×Z2 has elements ( 0 , 0 ) , ( 0 , 1 ) , ( 1 , 0 ) , ( 1 , 1 ) , ( 2 , 0 ) , ( 2 , 1 ) , ( 3 , 0 ) , ( 3 , 1 ) ( 0 , 0 ) , ( 0 , 1 ) , ( 1 , 0 ) , ( 1 , 1 ) , ( 2 , 0 ) , ( 2 , 1 ) , ( 3 , 0 ) , ( 3 , 1 ) (0,0),(0,1),(1,0),(1,1),(2,0),(2,1),(3,0),(3,1)(0,0), (0,1), (1,0), (1,1), (2,0), (2,1), (3,0), (3,1)(0,0),(0,1),(1,0),(1,1),(2,0),(2,1),(3,0),(3,1) and is not cyclic.
Since Z 8 Z 8 Z_(8)\mathbb{Z}_8Z8 is cyclic and Z 4 × Z 2 Z 4 × Z 2 Z_(4)xxZ_(2)\mathbb{Z}_4 \times \mathbb{Z}_2Z4×Z2 is not, they cannot be isomorphic.
Thus, the statement is false, as demonstrated by this counterexample.

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iv) If G G G\mathrm{G}G is a finite group and m o ( G ) , m N m o ( G ) , m N m∣o(G),minN\mathrm{m} \mid \mathrm{o}(\mathrm{G}), \mathrm{m} \in \mathbb{N}mo(G),mN, then G G G\mathrm{G}G has a subgroup of order m m m\mathrm{m}m.
Answer:
The statement “If G G GGG is a finite group and m o ( G ) , m N m o ( G ) , m N m∣o(G),m inNm \mid o(G), m \in \mathbb{N}mo(G),mN, then G G GGG has a subgroup of order m m mmm” is false.

Justification:

The statement would be true if G G GGG were a cyclic group, as every divisor of the order of a cyclic group corresponds to a unique subgroup. However, the statement is not generally true for all finite groups.

Counterexample:

Consider the symmetric group S 3 S 3 S_(3)S_3S3 with o ( S 3 ) = 6 o ( S 3 ) = 6 o(S_(3))=6o(S_3) = 6o(S3)=6. The divisors of 6 are 1 , 2 , 3 , 6 1 , 2 , 3 , 6 1,2,3,61, 2, 3, 61,2,3,6.
While S 3 S 3 S_(3)S_3S3 does have subgroups of orders 1, 2, 3, and 6, it does not have a subgroup of order 4, even though 4 is a natural number ( m N m N m inNm \in \mathbb{N}mN).
Thus, the statement is false, as demonstrated by this counterexample.

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v) If β 1 β 1 beta_(1)\beta_1β1 and β 2 β 2 beta_(2)\beta_2β2 are two 15 th 15 th  15^(“th “)15^{\text {th }}15th  roots of unity, then Q ( β 1 ) = Q ( β 2 ) Q β 1 = Q β 2 Q(beta_(1))=Q(beta_(2))\mathbb{Q}\left(\beta_1\right)=\mathbb{Q}\left(\beta_2\right)Q(β1)=Q(β2).
Answer:
The statement “If β 1 β 1 beta_(1)\beta_1β1 and β 2 β 2 beta_(2)\beta_2β2 are two 15 th 15 th 15^(“th”)15^{\text{th}}15th roots of unity, then Q ( β 1 ) = Q ( β 2 ) Q ( β 1 ) = Q ( β 2 ) Q(beta_(1))=Q(beta_(2))\mathbb{Q}(\beta_1) = \mathbb{Q}(\beta_2)Q(β1)=Q(β2)” is true.

Justification:

The 15 th 15 th 15^(“th”)15^{\text{th}}15th roots of unity are the complex numbers of the form
β k = cos ( 2 π k 15 ) + i sin ( 2 π k 15 ) , β k = cos 2 π k 15 + i sin 2 π k 15 , beta _(k)=cos((2pi k)/(15))+i sin((2pi k)/(15)),\beta_k = \cos\left(\frac{2\pi k}{15}\right) + i \sin\left(\frac{2\pi k}{15}\right),βk=cos(2πk15)+isin(2πk15),
where i i iii is the imaginary unit and k = 0 , 1 , 2 , , 14 k = 0 , 1 , 2 , , 14 k=0,1,2,dots,14k = 0, 1, 2, \ldots, 14k=0,1,2,,14.
These roots of unity are all solutions to the equation x 15 1 = 0 x 15 1 = 0 x^(15)-1=0x^{15} – 1 = 0x151=0, and they generate a cyclotomic field Q ( ζ 15 ) Q ( ζ 15 ) Q(zeta_(15))\mathbb{Q}(\zeta_{15})Q(ζ15), where ζ 15 = e 2 π i / 15 ζ 15 = e 2 π i / 15 zeta_(15)=e^(2pi i//15)\zeta_{15} = e^{2\pi i / 15}ζ15=e2πi/15.
Now, any 15 th 15 th 15^(“th”)15^{\text{th}}15th root of unity can be expressed as a power of ζ 15 ζ 15 zeta_(15)\zeta_{15}ζ15. Specifically, β 1 = ζ 15 k 1 β 1 = ζ 15 k 1 beta_(1)=zeta_(15)^(k_(1))\beta_1 = \zeta_{15}^{k_1}β1=ζ15k1 and β 2 = ζ 15 k 2 β 2 = ζ 15 k 2 beta_(2)=zeta_(15)^(k_(2))\beta_2 = \zeta_{15}^{k_2}β2=ζ15k2 for some k 1 , k 2 { 0 , 1 , 2 , , 14 } k 1 , k 2 { 0 , 1 , 2 , , 14 } k_(1),k_(2)in{0,1,2,dots,14}k_1, k_2 \in \{0, 1, 2, \ldots, 14\}k1,k2{0,1,2,,14}.
Since both β 1 β 1 beta_(1)\beta_1β1 and β 2 β 2 beta_(2)\beta_2β2 can be expressed using powers of ζ 15 ζ 15 zeta_(15)\zeta_{15}ζ15, it follows that Q ( β 1 ) Q ( β 1 ) Q(beta_(1))\mathbb{Q}(\beta_1)Q(β1) and Q ( β 2 ) Q ( β 2 ) Q(beta_(2))\mathbb{Q}(\beta_2)Q(β2) are both subfields of Q ( ζ 15 ) Q ( ζ 15 ) Q(zeta_(15))\mathbb{Q}(\zeta_{15})Q(ζ15).
Moreover, β 1 β 1 beta_(1)\beta_1β1 and β 2 β 2 beta_(2)\beta_2β2 generate the same field as ζ 15 ζ 15 zeta_(15)\zeta_{15}ζ15 because they are powers of ζ 15 ζ 15 zeta_(15)\zeta_{15}ζ15. Therefore, Q ( β 1 ) = Q ( ζ 15 ) Q ( β 1 ) = Q ( ζ 15 ) Q(beta_(1))=Q(zeta_(15))\mathbb{Q}(\beta_1) = \mathbb{Q}(\zeta_{15})Q(β1)=Q(ζ15) and Q ( β 2 ) = Q ( ζ 15 ) Q ( β 2 ) = Q ( ζ 15 ) Q(beta_(2))=Q(zeta_(15))\mathbb{Q}(\beta_2) = \mathbb{Q}(\zeta_{15})Q(β2)=Q(ζ15), which implies Q ( β 1 ) = Q ( β 2 ) Q ( β 1 ) = Q ( β 2 ) Q(beta_(1))=Q(beta_(2))\mathbb{Q}(\beta_1) = \mathbb{Q}(\beta_2)Q(β1)=Q(β2).
Thus, the statement is true.

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vi) There exists an extension field of Z 3 Z 3 Z_(3)\mathbb{Z}_3Z3 of order 25 .
Answer:
The statement “There exists an extension field of Z 3 Z 3 Z_(3)\mathbb{Z}_3Z3 of order 25″ is false.

Justification:

An extension field K K KKK of a finite field F F FFF of order p n p n p^(n)p^npn (where p p ppp is a prime and n n nnn is a positive integer) must have order p m p m p^(m)p^mpm for some integer m > n m > n m > nm > nm>n. This is because the extension field K K KKK can be thought of as a vector space over F F FFF, and the size of K K KKK must be p m p m p^(m)p^mpm to be compatible with the vector space structure.
In the given statement, Z 3 Z 3 Z_(3)\mathbb{Z}_3Z3 is a field of order 3 3 333, and we are asked about an extension field of order 25 25 252525. The number 25 = 5 2 25 = 5 2 25=5^(2)25 = 5^225=52 is not of the form 3 m 3 m 3^(m)3^m3m for any integer m m mmm.
Therefore, there cannot exist an extension field of Z 3 Z 3 Z_(3)\mathbb{Z}_3Z3 with order 25 25 252525.
Thus, the statement is false.

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vii) Every group of order 18 has a normal subgroup of order 2 .
Answer:
The statement “Every group of order 18 has a normal subgroup of order 2” is false.

Justification:

A group of order 18 has order 2 × 9 2 × 9 2xx92 \times 92×9, which is divisible by the primes 2 and 3. According to the Sylow theorems, for a group G G GGG of order 18 = 2 × 9 18 = 2 × 9 18=2xx918 = 2 \times 918=2×9, the number n 2 n 2 n_(2)n_2n2 of Sylow 2-subgroups must divide 9 and be congruent to 1 modulo 2. The possible values for n 2 n 2 n_(2)n_2n2 are therefore 1, 3, or 9.

Counterexample:

Consider the dihedral group D 9 D 9 D_(9)D_9D9, which is the group of symmetries of a regular 9-gon. The order of D 9 D 9 D_(9)D_9D9 is 18. In D 9 D 9 D_(9)D_9D9, the number of Sylow 2-subgroups is 9, and none of them are normal in D 9 D 9 D_(9)D_9D9.
Thus, D 9 D 9 D_(9)D_9D9 is a group of order 18 that does not have a normal subgroup of order 2, making the statement false.

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viii) If I I I\mathrm{I}I and J J J\mathrm{J}J are ideals of a ring R R R\mathrm{R}R, then I J = I J I J = I J IJ=InnJ\mathrm{IJ}=\mathrm{I} \cap \mathrm{J}IJ=IJ.
Answer:
The statement “If I I III and J J JJJ are ideals of a ring R R RRR, then I J = I J I J = I J IJ=I nn JIJ = I \cap JIJ=IJ” is false.

Justification:

  1. Product of Ideals I J I J IJIJIJ: The product I J I J IJIJIJ is defined as the set { i = 1 n a i b i a i I , b i J , n N } { i = 1 n a i b i a i I , b i J , n N } {sum_(i=1)^(n)a_(i)b_(i)∣a_(i)in I,b_(i)in J,n inN}\{ \sum_{i=1}^{n} a_i b_i \mid a_i \in I, b_i \in J, n \in \mathbb{N} \}{i=1naibiaiI,biJ,nN}. It is also an ideal of R R RRR.
  2. Intersection of Ideals I J I J I nn JI \cap JIJ: The intersection I J I J I nn JI \cap JIJ is the set of all elements that are both in I I III and J J JJJ. It is also an ideal of R R RRR.

Counterexample:

Consider the ring R = Z R = Z R=ZR = \mathbb{Z}R=Z of integers. Let I = 2 Z I = 2 Z I=2ZI = 2\mathbb{Z}I=2Z and J = 3 Z J = 3 Z J=3ZJ = 3\mathbb{Z}J=3Z.
  1. I J = { 2 × 3 , 2 × 6 , 2 × 9 , , 4 × 3 , 4 × 6 , } = 6 Z I J = { 2 × 3 , 2 × 6 , 2 × 9 , , 4 × 3 , 4 × 6 , } = 6 Z IJ={2xx3,2xx6,2xx9,dots,4xx3,4xx6,dots}=6ZIJ = \{ 2 \times 3, 2 \times 6, 2 \times 9, \ldots, 4 \times 3, 4 \times 6, \ldots \} = 6\mathbb{Z}IJ={2×3,2×6,2×9,,4×3,4×6,}=6Z
  2. I J = { n Z n is divisible by both 2 and 3 } = 6 Z I J = { n Z n  is divisible by both 2 and 3 } = 6 Z I nn J={n inZ∣n” is divisible by both 2 and 3″}=6ZI \cap J = \{ n \in \mathbb{Z} \mid n \text{ is divisible by both 2 and 3} \} = 6\mathbb{Z}IJ={nZn is divisible by both 2 and 3}=6Z
In this example, I J = I J I J = I J IJ=I nn JIJ = I \cap JIJ=IJ, but this is not generally true for all ideals I I III and J J JJJ in all rings R R RRR.
For instance, consider the ring R = Z [ x ] R = Z [ x ] R=Z[x]R = \mathbb{Z}[x]R=Z[x] of all polynomials with integer coefficients. Let I = ( 2 , x ) I = ( 2 , x ) I=(2,x)I = (2, x)I=(2,x) and J = ( 3 , x ) J = ( 3 , x ) J=(3,x)J = (3, x)J=(3,x).
  1. I J = ( 6 , 2 x , 3 x , x 2 ) I J = ( 6 , 2 x , 3 x , x 2 ) IJ=(6,2x,3x,x^(2))IJ = (6, 2x, 3x, x^2)IJ=(6,2x,3x,x2)
  2. I J = ( 6 , x ) I J = ( 6 , x ) I nn J=(6,x)I \cap J = (6, x)IJ=(6,x)
Here, I J I J I J I J IJ!=I nn JIJ \neq I \cap JIJIJ, disproving the statement.
Thus, the statement is false, as demonstrated by this counterexample.

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ix) If f : R S f : R S f:RrarrS\mathrm{f}: \mathrm{R} \rightarrow \mathrm{S}f:RS is a ring homomorphism and I I I\mathrm{I}I is an ideal of R R R\mathrm{R}R, then f ( I ) f ( I ) f(I)\mathrm{f}(\mathrm{I})f(I) is an ideal of S S S\mathrm{S}S.
Answer:
The statement “If f : R S f : R S f:R rarr Sf: R \rightarrow Sf:RS is a ring homomorphism and I I III is an ideal of R R RRR, then f ( I ) f ( I ) f(I)f(I)f(I) is an ideal of S S SSS” is false.

Justification:

An ideal I I III of a ring R R RRR is a subset of R R RRR that is closed under addition and multiplication by any element in R R RRR. A ring homomorphism f : R S f : R S f:R rarr Sf: R \rightarrow Sf:RS preserves addition and multiplication but does not necessarily map ideals to ideals.

Counterexample:

Consider the ring R = Z R = Z R=ZR = \mathbb{Z}R=Z and the ring S = Z / 2 Z S = Z / 2 Z S=Z//2ZS = \mathbb{Z}/2\mathbb{Z}S=Z/2Z (the integers modulo 2). Let f : R S f : R S f:R rarr Sf: R \rightarrow Sf:RS be the natural projection map defined by f ( x ) = x mod 2 f ( x ) = x mod 2 f(x)=xmod2f(x) = x \mod 2f(x)=xmod2.
Let I = 2 Z I = 2 Z I=2ZI = 2\mathbb{Z}I=2Z be the ideal of even integers in R R RRR.
The image f ( I ) f ( I ) f(I)f(I)f(I) consists of the set { 0 } { 0 } {0}\{0\}{0} in S S SSS, since all even integers are mapped to 0 in Z / 2 Z Z / 2 Z Z//2Z\mathbb{Z}/2\mathbb{Z}Z/2Z.
However, { 0 } { 0 } {0}\{0\}{0} is not an ideal in S S SSS because it is not closed under multiplication by any element in S S SSS. Specifically, 0 × 1 = 0 0 × 1 = 0 0xx1=00 \times 1 = 00×1=0 but 1 1 111 is not in { 0 } { 0 } {0}\{0\}{0}.
Thus, f ( I ) f ( I ) f(I)f(I)f(I) is not an ideal in S S SSS, making the statement false.

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x) Every prime ideal of an integral domain is a maximal ideal.
Answer:
The statement “Every prime ideal of an integral domain is a maximal ideal” is false.

Justification:

  1. Prime Ideal: An ideal P P PPP in a commutative ring R R RRR is called a prime ideal if for any a , b R a , b R a,b in Ra, b \in Ra,bR, a b P a b P ab in Pab \in PabP implies a P a P a in Pa \in PaP or b P b P b in Pb \in PbP.
  2. Maximal Ideal: An ideal M M MMM in a commutative ring R R RRR is called a maximal ideal if M R M R M!=RM \neq RMR and there is no ideal N N NNN such that M N R M N R M⊊N⊊RM \subsetneq N \subsetneq RMNR.

Counterexample:

Consider the integral domain R = Z [ x ] R = Z [ x ] R=Z[x]R = \mathbb{Z}[x]R=Z[x], the ring of all polynomials with integer coefficients.
Let P = ( x ) P = ( x ) P=(x)P = (x)P=(x), the ideal generated by x x xxx. This is a prime ideal because Z [ x ] / ( x ) Z Z [ x ] / ( x ) Z Z[x]//(x)~=Z\mathbb{Z}[x]/(x) \cong \mathbb{Z}Z[x]/(x)Z, which is an integral domain. However, P P PPP is not maximal because it is properly contained in other ideals, such as ( x , 2 ) ( x , 2 ) (x,2)(x, 2)(x,2), which in turn is properly contained in R R RRR.
Thus, P P PPP is a prime ideal that is not maximal, disproving the statement.
Therefore, the statement is false, as demonstrated by this counterexample.

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Question:-02

  1. a) Let G G G\mathrm{G}G be a group and let H G , K G , o ( H ) = o ( K ) = p H G , K G , o ( H ) = o ( K ) = p H <= G,K <= G,o(H)=o(K)=p\mathrm{H} \leq \mathrm{G}, \mathrm{K} \leq \mathrm{G}, \mathrm{o}(\mathrm{H})=\mathrm{o}(\mathrm{K})=\mathrm{p}HG,KG,o(H)=o(K)=p, a prime. Show that either H K = { e } H K = { e } HnnK={e}\mathrm{H} \cap \mathrm{K}=\{\mathrm{e}\}HK={e} or H = K H = K H=K\mathrm{H}=\mathrm{K}H=K. Is this result still true if p p p\mathrm{p}p is not a prime? Justify your answer.
Answer:

Part 1: o ( H ) = o ( K ) = p o ( H ) = o ( K ) = p o(H)=o(K)=po(H) = o(K) = po(H)=o(K)=p, a prime

Let G G GGG be a group and H , K G H , K G H,K <= GH, K \leq GH,KG with o ( H ) = o ( K ) = p o ( H ) = o ( K ) = p o(H)=o(K)=po(H) = o(K) = po(H)=o(K)=p, where p p ppp is a prime number.
  1. Case 1: H K { e } H K { e } H nn K!={e}H \cap K \neq \{ e \}HK{e}
    • If H K H K H nn KH \cap KHK contains an element other than the identity e e eee, then H K H K H nn KH \cap KHK must be a subgroup of both H H HHH and K K KKK (since the intersection of two subgroups is always a subgroup).
    • The order of H K H K H nn KH \cap KHK must divide the order of H H HHH and K K KKK by Lagrange’s theorem. Since o ( H ) = o ( K ) = p o ( H ) = o ( K ) = p o(H)=o(K)=po(H) = o(K) = po(H)=o(K)=p and p p ppp is a prime, the only divisors are 1 and p p ppp.
    • Since H K H K H nn KH \cap KHK contains an element other than e e eee, its order must be p p ppp, which means H K = H = K H K = H = K H nn K=H=KH \cap K = H = KHK=H=K.
  2. Case 2: H K = { e } H K = { e } H nn K={e}H \cap K = \{ e \}HK={e}
    • If H K H K H nn KH \cap KHK contains only the identity e e eee, then H K = { e } H K = { e } H nn K={e}H \cap K = \{ e \}HK={e}.
So, either H K = { e } H K = { e } H nn K={e}H \cap K = \{ e \}HK={e} or H = K H = K H=KH = KH=K.

Part 2: o ( H ) = o ( K ) o ( H ) = o ( K ) o(H)=o(K)o(H) = o(K)o(H)=o(K) not necessarily a prime

If p p ppp is not a prime, the result may not hold. For example, consider G = Z 12 G = Z 12 G=Z_(12)G = \mathbb{Z}_{12}G=Z12, H = { 0 , 4 , 8 } H = { 0 , 4 , 8 } H={0,4,8}H = \{ 0, 4, 8 \}H={0,4,8}, and K = { 0 , 6 , 12 } K = { 0 , 6 , 12 } K={0,6,12}K = \{ 0, 6, 12 \}K={0,6,12}. Here, o ( H ) = o ( K ) = 3 o ( H ) = o ( K ) = 3 o(H)=o(K)=3o(H) = o(K) = 3o(H)=o(K)=3, which is not a prime. We have H K = { 0 } H K = { 0 } H nn K={0}H \cap K = \{ 0 \}HK={0}, but H K H K H!=KH \neq KHK.
Thus, the result is not necessarily true if p p ppp is not a prime.

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b) Let G G G\mathrm{G}G be the group of all rigid motions of a plane and S S S\mathrm{S}S be the set of all rectangles in the plane. Show that G G G\mathrm{G}G acts on S S S\mathrm{S}S. Also obtain the orbit and stabiliser of a square under this action.
Answer:
To show that G G GGG acts on S S SSS, we need to satisfy two conditions:
  1. For each g G g G g in Gg \in GgG and s S s S s in Ss \in SsS, g s g s g*sg \cdot sgs is also in S S SSS.
  2. For each g 1 , g 2 G g 1 , g 2 G g_(1),g_(2)in Gg_1, g_2 \in Gg1,g2G and s S s S s in Ss \in SsS, ( g 1 g 2 ) s = g 1 ( g 2 s ) ( g 1 g 2 ) s = g 1 ( g 2 s ) (g_(1)*g_(2))*s=g_(1)*(g_(2)*s)(g_1 \cdot g_2) \cdot s = g_1 \cdot (g_2 \cdot s)(g1g2)s=g1(g2s).

Condition 1: Closure

A rigid motion in the plane consists of rotations, translations, and reflections. When we apply any of these to a rectangle, we still get a rectangle. Therefore, G G GGG is closed under the action on S S SSS.

Condition 2: Associativity

Let g 1 , g 2 g 1 , g 2 g_(1),g_(2)g_1, g_2g1,g2 be any two rigid motions and s s sss be any rectangle. Then ( g 1 g 2 ) s ( g 1 g 2 ) s (g_(1)*g_(2))*s(g_1 \cdot g_2) \cdot s(g1g2)s is the rectangle obtained by first applying g 2 g 2 g_(2)g_2g2 to s s sss and then applying g 1 g 1 g_(1)g_1g1 to the result. This is the same as first applying g 2 g 2 g_(2)g_2g2 to s s sss to get g 2 s g 2 s g_(2)*sg_2 \cdot sg2s and then applying g 1 g 1 g_(1)g_1g1 to g 2 s g 2 s g_(2)*sg_2 \cdot sg2s to get g 1 ( g 2 s ) g 1 ( g 2 s ) g_(1)*(g_(2)*s)g_1 \cdot (g_2 \cdot s)g1(g2s). Therefore, ( g 1 g 2 ) s = g 1 ( g 2 s ) ( g 1 g 2 ) s = g 1 ( g 2 s ) (g_(1)*g_(2))*s=g_(1)*(g_(2)*s)(g_1 \cdot g_2) \cdot s = g_1 \cdot (g_2 \cdot s)(g1g2)s=g1(g2s).
So both conditions are satisfied, and G G GGG acts on S S SSS.

Orbit and Stabilizer for a Square

Orbit

The orbit of a square under this action is the set of all rectangles that can be obtained by applying a rigid motion to the square. Since a square is a special case of a rectangle, any rigid motion (rotation, translation, reflection) will still produce a square. Therefore, the orbit of a square under this action is the set of all squares in the plane.

Stabilizer

The stabilizer of a square is the set of all rigid motions that leave the square invariant. This includes:
  1. The identity motion (doing nothing).
  2. 90-degree rotations about the center of the square.
  3. 180-degree rotations about the center of the square.
  4. 270-degree rotations about the center of the square.
  5. Reflections about the axes of symmetry of the square.
So, the stabilizer consists of these 8 rigid motions.
Thus, we have shown that G G GGG acts on S S SSS and have obtained the orbit and stabilizer of a square under this action.

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c) Let G G G\mathrm{G}G be a finite group and H H H\mathrm{H}H be a normal subgroup of G G G\mathrm{G}G. Prove that H = C x H = C x H=uuC_(x)\mathrm{H}=\cup \mathrm{C}_{\mathrm{x}}H=Cx where the C x C x C_(x)\mathrm{C}_{\mathrm{x}}Cx are all the distinct conjugacy classes of G G G\mathrm{G}G such that H C x H C x HnnC_(x)!=O/\mathrm{H} \cap \mathrm{C}_{\mathrm{x}} \neq \varnothingHCx.
Answer:
To prove that H = C x H = C x H=uuC_(x)H = \cup C_xH=Cx where C x C x C_(x)C_xCx are all the distinct conjugacy classes of G G GGG such that H C x H C x H nnC_(x)!=O/H \cap C_x \neq \varnothingHCx, we need to show two things:
  1. H C x H C x H sube uuC_(x)H \subseteq \cup C_xHCx
  2. C x H C x H uuC_(x)sube H\cup C_x \subseteq HCxH

1. H C x H C x H sube uuC_(x)H \subseteq \cup C_xHCx

Take any element h H h H h in Hh \in HhH. Since H H HHH is a normal subgroup of G G GGG, it is invariant under conjugation by elements of G G GGG. That is, for any g G g G g in Gg \in GgG, g 1 h g H g 1 h g H g^(-1)hg in Hg^{-1}hg \in Hg1hgH.
The conjugacy class C h C h C_(h)C_hCh of h h hhh in G G GGG is defined as C h = { g 1 h g g G } C h = { g 1 h g g G } C_(h)={g^(-1)hg∣g in G}C_h = \{ g^{-1}hg \mid g \in G \}Ch={g1hggG}. Since H H HHH is normal, C h H C h H C_(h)sube HC_h \subseteq HChH.
Therefore, h C h h C h h inC_(h)h \in C_hhCh and C h H C h H C_(h)nn H!=O/C_h \cap H \neq \varnothingChH. So, h C x h C x h in uuC_(x)h \in \cup C_xhCx.
This shows that H C x H C x H sube uuC_(x)H \subseteq \cup C_xHCx.

2. C x H C x H uuC_(x)sube H\cup C_x \subseteq HCxH

Take any element x x xxx such that C x H C x H C_(x)nn H!=O/C_x \cap H \neq \varnothingCxH. Let h C x H h C x H h inC_(x)nn Hh \in C_x \cap HhCxH.
The conjugacy class C x C x C_(x)C_xCx is defined as C x = { g 1 x g g G } C x = { g 1 x g g G } C_(x)={g^(-1)xg∣g in G}C_x = \{ g^{-1}xg \mid g \in G \}Cx={g1xggG}.
Since h C x h C x h inC_(x)h \in C_xhCx, there exists some g G g G g in Gg \in GgG such that h = g 1 x g h = g 1 x g h=g^(-1)xgh = g^{-1}xgh=g1xg.
Since h H h H h in Hh \in HhH and H H HHH is a normal subgroup, g 1 h g H g 1 h g H g^(-1)hg in Hg^{-1}hg \in Hg1hgH. But g 1 h g = x g 1 h g = x g^(-1)hg=xg^{-1}hg = xg1hg=x, so x H x H x in Hx \in HxH.
This shows that C x H C x H C_(x)sube HC_x \subseteq HCxH for every x x xxx such that C x H C x H C_(x)nn H!=O/C_x \cap H \neq \varnothingCxH.
Therefore, C x H C x H uuC_(x)sube H\cup C_x \subseteq HCxH.

Conclusion

Since H C x H C x H sube uuC_(x)H \subseteq \cup C_xHCx and C x H C x H uuC_(x)sube H\cup C_x \subseteq HCxH, we have H = C x H = C x H=uuC_(x)H = \cup C_xH=Cx where C x C x C_(x)C_xCx are all the distinct conjugacy classes of G G GGG such that H C x H C x H nnC_(x)!=O/H \cap C_x \neq \varnothingHCx.
This completes the proof.

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