# Where does the word “torsion” in algebra come from?

Torsion is used to refer to elements of finite order under some binary operation. It doesn't seem to bear any relation to the ordinary everyday use of the word or with its use in differential geometry (which relates back to the ordinary use of the word). So how did it acquire this usage in algebra?

I'm interested to understand the intuition behind why the word "torsion" was chosen for this notion, as well as when it was first used.

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The topologists on the site should be able to help. My recollection on this is a bit fuzzy, but I seem to recall that the term has something to do with elements in homology groups of a space(?), and that these elements have something to do with twisting in the space. Then again, I could be completely wrong, it's been a while. –  Chris Leary Feb 11 '13 at 21:42
The word torsion itself originated in the 15th century meaning a "wringing pain in the bowels," from the Latin torsionem - "wringing" or "gripping" - and tortionem - "torture" or "torment." Now, I don't know how this came to have its current meaning in algebra, but as a finite group theorist, I suspect that it can be attributed to a general hatred of my people, and move that the term be changed to "wicked awesome." –  Alexander Gruber Feb 11 '13 at 22:51
I've always thought it made a kind of twisted sense (pun intended.) "Torsion" being "the action of twisting, or turning a body spirally," and an element of finite order generating a cyclic subgroup, you see an element of finite order act by turning along this cycle. –  Kundor Feb 12 '13 at 1:28
@Alexander But even mathematically, torsion is a "wringing pain in the bowels" (pain in the but), often causing torturous proofs. –  Math Gems Feb 12 '13 at 1:48
@Kundor Yeah that's roughly the picture I had but then it struck me that nobody had ever explicitly said that to me. –  Dan Piponi Feb 12 '13 at 5:07

John Stillwell wrote that "the word 'torsion'entered the theory of abelian groups as a result of the derivation of the one-dimensional torsion coefficients by abelianization of the fundamental group in Tietze 1908" [Classical Topology and Combinatorial Group Theory, 1993, Sec. 5.1.1, p. 170]. Below is an excerpt providing further context.

The appropriate notions of "sum" and "boundary,"and the correct choice of k-dimensional manifolds admissible as basis elements, were found only after considerable trial and error. "Appropriate" initially meant satisfying the relation $B_k = B_{m-k}$ since this was the relation Poincare tried to prove in his 1895 paper. Heegaard 1898 showed this work to be in error by constructing a counterexample. Poincare then changed the definition and proved the theorem again in Poincare 1899, inventing the tool of simplicial decomposition for the purpose. He also made a thorough analysis of his error, uncovering the important concept of torsion in Poincare 1900, and exposing the breakdown of his earlier proof as failure to observe torsion.

Torsion is present when an element a does not form a boundary taken once, but does when taken more than once. An example is the curve $a$ in the projective plane $P$ which generates $\pi_1(P).$ Then $a^2$ is the boundary of a disc, though a itself does not separate $P.$ Poincare justified the term "torsion" by showing that $(m-1)$-dimensional torsion is present only in an $m$-manifold which is nonorientable, and hence twisted onto itself in some sense.

In his first topology paper, Poincare 1892 showed that the Betti numbers alone did not determine a manifold up to homeomorphism. By 1900 he was hoping that torsion numbers would supply the missing information, and his paper of that year contains a decomposition of the homology in- formation in each dimension $k$ into the Betti number $B_k$ and a finite set of numbers called $k$-dimensional torsion coefficients. Since Noether 1926 it has been customary to encode this information in an abelian group $H_k$ called the $k$-dimensional homology group, and Poincare's construction can in fact be seen as the decomposition of a finitely generated abelian group into cyclic factors (see the structure theorem 5.2). The word "torsion," which appears so inexplicably in most algebra texts, entered the theory of abelian groups as a result of the derivation of the one-dimensional torsion coefficients by abelianization of the fundamental group in Tietze 1908 (see 5.1.3. and 5.3).

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There is an entry here at the "Earliest known uses" site that is sometimes useful. Quoting directly from it, for readers' benefit:

TORSION as used in group theory: an element of a group G is a torsion element if it generates a finite subgroup of G. An abelian group consisting entirely of torsion elements is called a torsion group. In any abelian group, the torsion elements form a subgroup, frequently called the torsion subgroup of G. An abelian group is torsion-free if the neutral element is its only torsion element.

This terminology seems to have arisen around 1930. Its origin lies in algebraic (or combinatorial) topology. Poincaré (Second complément à l´Analysis Situs, Proc. London Math. Soc, vol. 32 (1900), 277-308) defined torsion coefficients for manifolds (variétés), and he distinguished manifolds with and without torsion. In a later terminology, his torsion coefficients are structure constants of homology groups. In 1935, the textbook Topologie I by Alexandroff-Hopf has the following concept of torsion: “The elements of finite order of the r-th Betti group of E form a subgroup called the r-th torsion group of E.” Here, the use of the word torsion group is still tied to a topological context even though the concept of a torsion subgroup seems to be hinted at. In the group theoretical chapter of this book, the word torsion is not used. But in the same year 1935, the paper Countable torsion groups by Leon Zippin (Annals of Math. 36, 86-99) contains the following definition: “A torsion group T is a discrete countable abelian group, every element of which is of finite order” - quite the modern definition, except for the restriction to countable groups. Two years before, Ulm (Zur Theorie der abzählbar-unendlichen Abelschen Gruppen, Math. Annalen 107, pp. 774-803) did not yet use these terms, writing instead “groups all of whose elements have finite exponents.” The torsion terminology was slow in obtaining general acceptance; it was frequently used in research papers in the 1940s, and is applied consistently in Kaplansky’s Infinite Abelian Groups of 1954, while on the other hand, Marshall Hall’s textbook on group theory, published in 1959, introduces the term “periodic group,” explaining that “the term torsion group is used in certain applications.” The influential book on topological groups, Abstract Harmonic Analysis I by Hewitt and Ross (1963) uses the torsion terminology and may have been important in promoting it.

[This entry was contributed by Peter Flor.]

End quote. It's an interesting site!

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I wonder why the first paragraph excludes non-abelian torsion groups. –  Martin Brandenburg Feb 11 '13 at 23:14
@MartinBrandenburg It's probably because they are not part of the standard torsion landscape: modules over integral domains. Usually you want to talk about the set of torsion elements forming a subgroup, and if that subgroup is the whole group, then the group is torsion, but they probably don't always form a subgroup in not Abelian groups, and so people aren't as excited about it. That's not saying the definition in nonAbelian groups isn't interesting! I looked at unpopular definitions of torsion in modules during my dissertation... –  rschwieb Feb 12 '13 at 1:52
This terminology comes from algebraic topology. To each space $X$, a sequence of abelian groups is assigned, called homology groups, and if $X$ is “twisted,” then there are elements of finite order in some of these groups.
$\mathbb{R P}^2$ has $\mathbb{Z} / 2 \mathbb{Z}$ as its $H_2$, reflecting its non-orientability. –  Zhen Lin Feb 11 '13 at 22:18