How many proper nontrivial subgroups do D5 have?

2.5k Views Asked by At

Do I have to find out every element in D5 and draw a table to find out subgroups?

I know how to find out every single element in D5,

but can't think of how to find proper nontrivial subgroups

1

There are 1 best solutions below

0
On

There are five subgroups of order 2 consisting of $R_0$(rotation by zero degrees) and a flip $F_i\,\,(i\in \{1,2,3,4,5\}$).

Note that there is a subgroup of rotations (of order $5$) and hence generated by each non-identity rotation.

Claim There is no other non-trivial subgroup. (why?)

Hint

(1) What happens to a subgroup containing two flips(flips along different axes)?

(2) What happens to a subgroup containing a non-trivial (hence all) rotation and a flip?