OFFSET
1,4
EXAMPLE
For n=4, by the fundamental theorem of finite abelian groups, the group is either Z4 or Z2 x Z2. When you choose 2 random elements, if 1 element comes out of the 2 elements of order 4, you will know you have Z4. If the 2 elements are of order 2 in Z2 x Z2, you will know you have Z2 x Z2. Calculating the probabilities, when you choose 2 random elements, if the group is Z4, there is a 5/6 chance of knowing it. If it is Z2 x Z2, there is a 1/2 chance of knowing it. Since we assume each non-isomorphic abelian group of order n has the same probability of being the group, averaging 5/6 and 1/2 we get a 2/3 chance that the group is known after choosing 2 elements. Since the probability that a single random element will allow us to identify the group is 1/4, which is not greater than 1/2, a(4) = 2.
CROSSREFS
KEYWORD
nonn
AUTHOR
Isaac Lambert, Oct 30 2010
EXTENSIONS
Edited and terms a(16) onward added by Max Alekseyev, Oct 07 2023
STATUS
approved