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Algebra Difficulty 7.7 National olympiad, round 2 Find the answer

Is there a finite abelian group GG such that the product of the
orders of all its elements is 220092^{2009}?

A number or a short expression. Spacing and $ signs are ignored.

Solution

No, there is no such group.
By the structure theorem for finitely generated abelian groups,
GG can be written as a product of cyclic groups.
If any of these factors has odd order, then GG has an element of odd order,
so the product of the orders of all of its elements cannot be a power of 2.

We may thus consider only abelian 22-groups hereafter.
For such a group GG, the product of the orders of all of its elements
has the form 2k(G)2^{k(G)} for some nonnegative integer GG, and we must show
that it is impossible to achieve k(G)=2009k(G) = 2009.
Again by the structure theorem, we may write
Gi=1(\ZZ/2i\ZZ)ei G \cong \prod_{i=1}^\infty (\ZZ/2^i \ZZ)^{e_i}
for some nonnegative integers e1,e2,e_1,e_2,\dots, all but finitely many of
which are 00.

For any nonnegative integer mm, the elements of GG of order at most 2m2^m
form a subgroup isomorphic to
i=1(\ZZ/2min{i,m}\ZZ)ei, \prod_{i=1}^\infty (\ZZ/2^{\min\{i,m\}} \ZZ)^{e_i},
which has 2sm2^{s_m} elements for sm=i=1min{i,m}eis_m = \sum_{i=1}^\infty \min\{i,m\} e_i.
Hence
k(G)=i=1i(2si2si1). k(G) = \sum_{i=1}^\infty i(2^{s_i} - 2^{s_{i-1}}).
Since s1s2s_1 \leq s_2 \leq \cdots, k(G)+1k(G)+1 is always divisible by 2s12^{s_1}.
In particular, k(G)=2009k(G) = 2009 forces s11s_1 \leq 1.

However, the only cases where s11s_1 \leq 1 are where all of the eie_i are 00,
in which case k(G)=0k(G) = 0, or where ei=1e_i = 1 for some ii and ej=0e_j = 0
for jij \neq i, in which case k(G)=(i1)2i+1k(G) = (i-1)2^i + 1.
The right side is a strictly increasing function
of ii which equals 17931793 for i=8i=8 and 40974097 for i=9i=9, so it can never equal
2009. This proves the claim.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.