Minimum size of generating set of subgroup may be more than of whole group
- Every finite group is a subgroup of a finite 2-generated group: This gives infinitely many examples, when we combine it with the fact that there are finite groups with arbitrarily large minimum size of generating set.
- Cyclicity is subgroup-closed: In particular, this means that any group where the minimum size of generating set is bigger than 1 cannot be put in a group where the minimum size of generating set is 1 or less.
Counterexample with smallest order
This is the smallest order counterexample both for and for .
Let be SmallGroup(16,3), i.e., it is a group of order 16 given by the presentation:
Note that although the above presentation uses three generators, the minimum size of generating set for is 2, because it is generated by and alone, because by the final relation in the presentation.
Consider the subgroup:
It is easy to check that is isomorphic to elementary abelian group:E8, and in particular its minimum size of generating set is 3, which is bigger than that of 2.
Explanation for why no example of smaller order is possible: Because cyclicity is subgroup-closed, it is not possible to get an example with cyclic. Thus, must have a minimum size of generating set of size at least 2. To yield a counterexample, must have a minimum size of generating set at least 3. We know that the minimum size of generating set is bounded from above by the sum of exponents of the prime divisors in the prime factorization of the order, so the smallest possible order allowing a minimum size of generating set is . The only group of this order satisfying the necessary condition is elementary abelian group:E8. Since the order of divides the order of , but they cannot be equal, the order of is at least . Of the groups of order 16 that contain this as a subgroup, SmallGroup(16,3) is the only one having a generating set of size two.
Construction of general family of counterexamples
Fact (1), along with the observation that we can construct finite groups with arbitrarily large minimum size of generating sets (e.g., elementary abelian groups of arbitrarily large rank), gives us a large and infinite colleciton of examples.
Fact (1) is typically proved using symmetric group on a finite set as the large group in which the embedding is done. The smallest example of such an embedding is the embedding of elementary abelian group:E8 in symmetric group:S6 (this is not the Cayley embedding; the Cayley embedding would be in symmetric group:S8).