# 2.3 Subgroups

1.

a. No, 1 + 1 ∉ H.

c. No, 3

^{2}= 9 ∉ H.e. No, (1 2)(3 4) · (1 3)(2 4) = (1 4)(2 3) ∉ H.

g. Yes, 0 = 6 H. H is closed because it consists of the even residues in ; −4 = 2, −2 = 4, so it is closed under inverses.

i. Yes, the unity (0, 0) H. If (m, k) and (m′, k′) are in H, then so is (m, k) + (m′, k′) = (m + m′, k + k′) and −(m, k) = (− m, − k).

3. Yes. If H is a subgroup of G and K is a subgroup of H, then 1 K (it is the unity of H). If a, b K, then ab K because this is their product in H. Finally, a

^{−1}is the inverse of a in H, hence in K.5. a. We have 1 H because 1 = 1

^{2}. If a, b H, then a^{−1}= a (because a^{2}= 1), so a^{−1}H. Finally, the fact that ab = ba gives (Get *Introduction to Abstract Algebra, Solutions Manual, 4th Edition* now with O’Reilly online learning.

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