- #1
RJLiberator
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Homework Statement
Prove or disprove each of the following:
a) {f ∈ S_n | f(1) = 1} is a subgroup of Sn
b) {e}∪{f∈D_n | f is a rotation} is a subgroup of D_n
c) {e} ∪ { f∈D_n | f is a reflection} is a subgroup of D_n
Homework Equations
e = identity element
Subgroup if:
i) e exists (identity)
ii) if x, y exist in the set then x*y exists
iii) for all x existing in the set, there is an inverse existing.
The Attempt at a Solution
a) we can prove that this is indeed a subgroup by following the subgroup definition
1. the identity is 1 and that exists in f(1) = 1.
2. For everything that exists in f(1)=1, 1*1 = 1. That exists still.
3. Since 1 is the only thing that exists in this set, 1*1 = 1 which is also the inverse element.
So yes, this is a subgroup of Sn.
b) 1. The identity clearly is part of this set.
2. since all elements are rotations, then they all exist when you multiply each of them. rot1*rot2 = just some other rotation?
3. Wouldn't the inverse be the opposite rotation?
c) 1. The identity clearly is part of this set.
2. the reflections all exist, if you multiply them, theyll just be reflections that still exist.
3. Wouldn't the inverse be the opposite reflection?Is my thinking correct?
Thank you.