Refinement
The condition Ext1(A, Z) = 0 can be equivalently formulated as follows: whenever B is an abelian group and f : B → A is a surjective group homomorphism whose kernel is isomorphic to the group of integers Z, then there exists a group homomorphism g : A → B with fg = idA. Abelian groups satisfying this condition are sometimes called Whitehead groups, so Whitehead's problem asks: is every Whitehead group free?
Caution: The converse of Whitehead's problem, namely that every free abelian group is Whitehead, is a well known group-theoretical fact. Some authors call Whitehead group only a non-free group A satisfying Ext1(A, Z) = 0. Whitehead's problem then asks: do Whitehead groups exist?
Read more about this topic: Whitehead Problem
Famous quotes containing the word refinement:
“It is an immense loss to have all robust and sustaining expletives refined away from one! At ... moments of trial refinement is a feeble reed to lean upon.”
—Alice James (1848–1892)
“Perhaps our own woods and fields,—in the best wooded towns, where we need not quarrel about the huckleberries,—with the primitive swamps scattered here and there in their midst, but not prevailing over them, are the perfection of parks and groves, gardens, arbors, paths, vistas, and landscapes. They are the natural consequence of what art and refinement we as a people have.... Or, I would rather say, such were our groves twenty years ago.”
—Henry David Thoreau (1817–1862)