Definition of height
Let ''A'' be an abelian group and ''g'' an element of ''A''. The ''p''-height of ''g'' in ''A'', denoted ''h''''p''(''g''), is the largest natural number ''n'' such that the equation ''p''''n''''x'' = ''g'' has a solution in ''x'' ∈ ''A'', or the symbol ∞ if a solution exists for all ''n''. Thus ''h''''p''(''g'') = ''n'' if and only if ''g'' ∈ ''p''''n''''A'' and ''g'' ∉ ''p''''n''+1''A''. This allows one to refine the notion of height. For any ordinal ''α'', there is a subgroup ''p''''α''''A'' of ''A'' which is the image of the multiplication map by ''p'' iterated ''α'' times, defined using transfinite induction: * ''p''0''A'' = ''A''; * ''p''''α''+1''A'' = ''p''(''p''''α''''A''); * ''p''''β''''A''=∩''α'' < ''β'' ''p''''α''''A'' if ''β'' is a limit ordinal. The subgroups ''p''''α''''A'' form a decreasing filtration of the group ''A'', and their intersection is the subgroup of the ''p''-divisible elements of ''A'', whose elements are assigned height ∞. The modified ''p''-height ''h''''p''∗(''g'') = ''α'' if ''g'' ∈ ''p''''α''''A'', but ''g'' ∉ ''p''''α''+1''A''. The construction of ''p''''α''''A'' is functorial in ''A''; in particular, subquotients of the filtration are isomorphism invariants of ''A''.Ulm subgroups
Let ''p'' be a fixed prime number. The (first) Ulm subgroup of an abelian group ''A'', denoted ''U''(''A'') or ''A''1, is ''p''''ω''''A'' = ∩''n'' ''p''''n''''A'', where ''ω'' is the smallest infinite ordinal. It consists of all elements of ''A'' of infinite height. The family of Ulm subgroups indexed by ordinals ''σ'' is defined by transfinite induction: * ''U''0(''A'') = ''A''; * ''U''''σ''+1(''A'') = ''U''(''U''''σ''(''A'')); * ''U''''τ''(''A'') = ∩''σ'' < ''τ'' ''U''''σ''(''A'') if ''τ'' is a limit ordinal. Equivalently, ''U''''σ''(''A'') = ''p''''ωσ''''A'', where ''ωσ'' is the product of ordinals ''ω'' and ''σ''. Ulm subgroups form a decreasing filtration of ''A'' whose quotients ''U''''σ''(''A'') = ''U''''σ''(''A'')/''U''''σ''+1(''A'') are called the Ulm factors of ''A''. This filtration stabilizes and the smallest ordinal ''τ'' such that ''U''''τ''(''A'') = ''U''''τ''+1(''A'') is the Ulm length of ''A''. The smallest Ulm subgroup ''U''''τ''(''A''), also denoted ''U''∞(''A'') and ''p''∞A, consists of all ''p''-divisible elements of ''A'', and being divisible group, it is a direct summand of ''A''. For every Ulm factor ''U''''σ''(''A'') the ''p''-heights of its elements are finite and they are unbounded for every Ulm factor except possibly the last one, namely ''U''''τ''−1(''A'') when the Ulm length ''τ'' is a successor ordinal.Ulm's theorem
The second Prüfer theorem provides a straightforward extension of the fundamental theorem of finitely generated abelian groups to countable abelian ''p''-groups without elements of infinite height: each such group is isomorphic to a direct sum of cyclic groups whose orders are powers of ''p''. Moreover, the cardinality of the set of summands of order ''p''''n'' is uniquely determined by the group and each sequence of at most countable cardinalities is realized. Helmut Ulm (1933) found an extension of this classification theory to general countable ''p''-groups: their isomorphism class is determined by the isomorphism classes of the Ulm factors and the ''p''-divisible part. : Ulm's theorem. ''Let'' ''A'' ''and'' ''B'' ''be countable abelian'' ''p''-''groups such that for every ordinal'' ''σ'' ''their Ulm factors are isomorphic'', ''U''''σ''(''A'') ≅ ''U''''σ''(''B'') ''and the'' ''p''-''divisible parts of'' ''A'' ''and'' ''B'' ''are isomorphic'', ''U''∞(''A'') ≅ ''U''∞(''B''). ''Then'' ''A'' ''and'' ''B'' ''are isomorphic.'' There is a complement to this theorem, first stated by Leo Zippin (1935) and proved in Kurosh (1960), which addresses the existence of an abelian ''p''-group with given Ulm factors. : ''Let'' ''τ'' ''be an ordinal and'' ''be a family of countable abelian'' ''p''-''groups indexed by the ordinals'' ''σ'' < ''τ'' ''such that the'' ''p''-''heights of elements of each'' ''A''''σ'' ''are finite and, except possibly for the last one, are unbounded. Then there exists a reduced abelian'' ''p''-''group'' ''A'' ''of Ulm length'' ''τ'' ''whose Ulm factors are isomorphic to these'' ''p''-''groups'', ''U''''σ''(''A'') ≅ ''A''''σ''. Ulm's original proof was based on an extension of the theory of elementary divisors to infinite matrices.Alternative formulation
George Mackey and Irving Kaplansky generalized Ulm's theorem to certain modules over aReferences
* László Fuchs (1970), ''Infinite abelian groups, Vol. I''. Pure and Applied Mathematics, Vol. 36. New York–London: Academic Press * Irving Kaplansky and George Mackey, ''A generalization of Ulm's theorem''. Summa Brasil. Math. 2, (1951), 195–202 * * {{cite journal , last1 = Ulm , first1 = H , year = 1933 , title = Zur Theorie der abzählbar-unendlichen Abelschen Gruppen , journal = Math. Ann. , volume = 107 , pages = 774–803 , JFM=59.0143.03 , doi=10.1007/bf01448919 Abelian group theory Infinite group theory