# Difference between revisions of "Whitehead problem"

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+ | $#C+1 = 23 : ~/encyclopedia/old_files/data/W110/W.1100030 Whitehead problem | ||

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− | In 1973, S. Shelah [[#References|[a5]]] proved that it is undecidable under the axioms of Zermelo–Frankel set theory (cf. also [[Set theory|Set theory]]), ZFC, whether every Whitehead group of cardinality | + | {{TEX|auto}} |

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+ | A problem attributed, to J.H.C. Whitehead, which asks for a characterization of Abelian groups $ A $ | ||

+ | that satisfy the homological condition $ { \mathop{\rm Ext} } ( A, \mathbf Z ) = 0 $, | ||

+ | where $ \mathbf Z $ | ||

+ | is the group of integers under addition (cf. also [[Homology|Homology]]). A group which satisfies this condition is called a Whitehead group. An equivalent characterization is: $ A $ | ||

+ | is a Whitehead group if and only if for every surjective homomorphism $ \pi $ | ||

+ | onto $ A $, | ||

+ | if the kernel of $ \pi $ | ||

+ | is isomorphic to $ \mathbf Z $, | ||

+ | then the kernel is a direct summand of the domain of $ \pi $. | ||

+ | A sufficient condition for $ A $ | ||

+ | to be a Whitehead group is that $ A $ | ||

+ | is free (see [[Free Abelian group|Free Abelian group]]). This condition has been proved to be necessary if $ A $ | ||

+ | is countable. Since a subgroup of a Whitehead group is again a Whitehead group, this means that every Whitehead group is $ \aleph _ {1} $- | ||

+ | free, that is, every countable subgroup is free. Before 1973 only partial results were obtained for uncountable groups: Whitehead groups were proved to be separable and slender, and under the assumption of the [[Continuum hypothesis|continuum hypothesis]] ( $ { \mathop{\rm CH} } $), | ||

+ | they were proved to be strongly $ \aleph _ {1} $- | ||

+ | free, that is, every countable subset is contained in a countable free subgroup which is a direct summand of countable subgroups containing it (see [[#References|[a3]]] for results prior to 1973). | ||

+ | |||

+ | In 1973, S. Shelah [[#References|[a5]]] proved that it is undecidable under the axioms of Zermelo–Frankel set theory (cf. also [[Set theory|Set theory]]), ZFC, whether every Whitehead group of cardinality $ \aleph _ {1} $ | ||

+ | is free. Specifically, he proved that this is true assuming the Gödel constructibility axiom, $ V = L $( | ||

+ | see [[Gödel constructive set|Gödel constructive set]]), but it is false assuming Martin's axiom (cf. [[Suslin hypothesis|Suslin hypothesis]]) and the negation of the continuum hypothesis ( $ { \mathop{\rm MA} } + \neg { \mathop{\rm CH} } $). | ||

+ | Later he proved that $ V = L $ | ||

+ | implies that every Whitehead group, of arbitrary cardinality, is free [[#References|[a6]]]. He also proved that the problem is undecidable even if $ { \mathop{\rm CH} } $ | ||

+ | is assumed [[#References|[a7]]] and that it is consistent with $ \neg { \mathop{\rm CH} } $ | ||

+ | that there are Whitehead groups of cardinality $ \aleph _ {1} $ | ||

+ | that are not strongly $ \aleph _ {1} $- | ||

+ | free [[#References|[a8]]]. | ||

For more information see [[#References|[a1]]], [[#References|[a4]]] or [[#References|[a2]]]. | For more information see [[#References|[a1]]], [[#References|[a4]]] or [[#References|[a2]]]. |

## Latest revision as of 08:29, 6 June 2020

A problem attributed, to J.H.C. Whitehead, which asks for a characterization of Abelian groups $ A $
that satisfy the homological condition $ { \mathop{\rm Ext} } ( A, \mathbf Z ) = 0 $,
where $ \mathbf Z $
is the group of integers under addition (cf. also Homology). A group which satisfies this condition is called a Whitehead group. An equivalent characterization is: $ A $
is a Whitehead group if and only if for every surjective homomorphism $ \pi $
onto $ A $,
if the kernel of $ \pi $
is isomorphic to $ \mathbf Z $,
then the kernel is a direct summand of the domain of $ \pi $.
A sufficient condition for $ A $
to be a Whitehead group is that $ A $
is free (see Free Abelian group). This condition has been proved to be necessary if $ A $
is countable. Since a subgroup of a Whitehead group is again a Whitehead group, this means that every Whitehead group is $ \aleph _ {1} $-
free, that is, every countable subgroup is free. Before 1973 only partial results were obtained for uncountable groups: Whitehead groups were proved to be separable and slender, and under the assumption of the continuum hypothesis ( $ { \mathop{\rm CH} } $),
they were proved to be strongly $ \aleph _ {1} $-
free, that is, every countable subset is contained in a countable free subgroup which is a direct summand of countable subgroups containing it (see [a3] for results prior to 1973).

In 1973, S. Shelah [a5] proved that it is undecidable under the axioms of Zermelo–Frankel set theory (cf. also Set theory), ZFC, whether every Whitehead group of cardinality $ \aleph _ {1} $ is free. Specifically, he proved that this is true assuming the Gödel constructibility axiom, $ V = L $( see Gödel constructive set), but it is false assuming Martin's axiom (cf. Suslin hypothesis) and the negation of the continuum hypothesis ( $ { \mathop{\rm MA} } + \neg { \mathop{\rm CH} } $). Later he proved that $ V = L $ implies that every Whitehead group, of arbitrary cardinality, is free [a6]. He also proved that the problem is undecidable even if $ { \mathop{\rm CH} } $ is assumed [a7] and that it is consistent with $ \neg { \mathop{\rm CH} } $ that there are Whitehead groups of cardinality $ \aleph _ {1} $ that are not strongly $ \aleph _ {1} $- free [a8].

For more information see [a1], [a4] or [a2].

#### References

[a1] | P.C. Eklof, "Whitehead's problem is undecidable" Amer. Math. Monthly , 83 (1976) pp. 775–788 |

[a2] | P.C. Eklof, A.H. Mekler, "Almost free modules" , North-Holland (1990) |

[a3] | L. Fuchs, "Infinite Abelian groups" , 2 , Acad. Press (1973) |

[a4] | R. J. Nunke, "Whitehead's problem" , Abelian Group Theory , Lecture Notes in Mathematics , 616 , Springer (1977) pp. 240–250 |

[a5] | S. Shelah, "Infinite abelian groups, Whitehead problem and some constructions" Israel J. Math. , 18 (1974) pp. 243–25 |

[a6] | S. Shelah, "A compactness theorem for singular cardinals, free algebras, Whitehead problem and transversals" Israel J. Math. , 21 (1975) pp. 319–349 |

[a7] | S. Shelah, "Whitehead groups may not be free even assuming CH, I" Israel J. Math. , 28 (1977) pp. 193–203 |

[a8] | S. Shelah, "On uncountable abelian groups" Israel J. Math. , 32 (1979) pp. 311–330 |

[a9] | S. Shelah, "Whitehead groups may not be free even assuming CH, II" Israel J. Math. , 35 (1980) pp. 257–285 |

**How to Cite This Entry:**

Whitehead problem.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Whitehead_problem&oldid=49210