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In [[mathematics]], the '''Herbrand quotient''' is a [[quotient]] of orders of [[Group cohomology|cohomology]] groups of a [[cyclic group]]. It was invented by [[Jacques Herbrand]].  It has an important application in [[class field theory]].

==Definition==
If ''G'' is a finite cyclic group acting on a [[G-module|''G''-module]] ''A'', then the cohomology groups ''H''<sup>''n''</sup>(''G'',''A'') have period 2 for ''n''≥1; in other words
:''H''<sup>''n''</sup>(''G'',''A'') = ''H''<sup>''n''+2</sup>(''G'',''A''),
an [[isomorphism]] induced by [[cup product]] with a generator of ''H''<sup>''2''</sup>(''G'','''Z''').  (If instead we use the [[Tate cohomology group]]s then the periodicity extends down to ''n''=0.)

A '''Herbrand module''' is an ''A'' for which the cohomology groups are finite.  In this case, the '''Herbrand quotient''' ''h''(''G'',''A'') is defined to be the quotient
:''h''(''G'',''A'') = |''H''<sup>''2''</sup>(''G'',''A'')|/|''H''<sup>''1''</sup>(''G'',''A'')|
of the order of the even and odd cohomology groups. 

===Alternative definition===
The quotient may be defined for a pair of [[endomorphism]]s of an [[Abelian group]], ''f'' and ''g'', which satisfy the condition ''fg'' = ''gf'' = 0.  Their Herbrand quotient ''q''(''f'',''g'') is defined as

:<math> q(f,g) = \frac{|\mathrm{ker} f:\mathrm{im} g|}{|\mathrm{ker} g:\mathrm{im} f|} </math>

if the two [[index of a group|indices]] are finite.  If ''G'' is a cyclic group with generator γ acting on an Abelian group ''A'', then we recover the previous definition by taking ''f'' = 1 - γ and ''g'' = 1 + γ + γ<sup>2</sup> + ... .

==Properties==
*The Herbrand quotient is [[multiplicative function|multiplicative]] on [[short exact sequence]]s.<ref name=C245>Cohen (2007) p.245</ref> In other words, if
:0 → ''A'' → ''B'' → ''C'' → 0
is exact, and any two of the quotients are defined, then so is the third and<ref name=S134>Serre (1979) p.134</ref>
:''h''(''G'',''B'')  = ''h''(''G'',''A'')''h''(''G'',''C'')
*If ''A'' is finite then ''h''(''G'',''A'') = 1.<ref name=S134/>
*For ''A'' is a submodule of the ''G''-module ''B'' of finite index, if either quotient is defined then so is the other and they are equal:<ref name=C245/> more generally, if there is a ''G''-morphism ''A'' → ''B'' with finite kernel and cokernel then the same holds.<ref name=S134/>
*If '''Z''' is the integers with ''G'' acting trivially, then ''h''(''G'','''Z''') = |''G''|
*If ''A'' is a finitely generated ''G''-module, then the Herbrand quotient ''h''(''A'') depends only on the complex ''G''-module '''C'''⊗''A'' (and so can be read off from the character of this complex representation of ''G'').

These properties mean that the Herbrand quotient is usually relatively easy to calculate, and is often much easier to calculate than the orders of either of the individual cohomology groups.

==See also==
*[[Class formation]]

==References==
{{reflist}}
* {{cite book | first1=M.F. | last1=Atiyah | author1-link=M. F. Atiyah | first2=C.T.C. | last2=Wall | author2-link=C. T. C. Wall | chapter=Cohomology of Groups | editor1-first=J.W.S. | editor1-last=Cassels | editor1-link=J. W. S. Cassels | editor2-first=Albrecht | editor2-last=Fröhlich | editor2-link=Albrecht Fröhlich | title=Algebraic Number Theory | year=1967 | publisher=Academic Press | zbl=0153.07403 }}  See section 8.
* {{cite book | first1=Emil | last1=Artin | author1-link=Emil Artin | first2=John | last2=Tate |author2-link=John Tate | title=Class Field Theory | publisher=AMS Chelsea | year=2009 | isbn=0-8218-4426-1 | zbl=1179.11040 | page=5 }}
* {{cite book |last=Cohen |first=Henri |authorlink= Henri Cohen (number theorist) |year=2007 |title=Number Theory – Volume I: Tools and Diophantine Equations|isbn= 978-0-387-49922-2|publisher=[[Springer-Verlag]]|series=[[Graduate Texts in Mathematics]]|volume=239| zbl=1119.11001 | pages=242–248}}
* {{cite book | first=Gerald J. | last=Janusz | title=Algebraic number fields | series=Pure and Applied Mathematics | volume=55 | publisher=Academic Press | year=1973 | page=142 | zbl=0307.12001 }}
* {{cite book | first=Helmut | last=Koch | title=Algebraic Number Theory | publisher=[[Springer-Verlag]] | year=1997 | isbn=3-540-63003-1 | zbl=0819.11044 | series=Encycl. Math. Sci. | volume=62 | edition=2nd printing of 1st | pages=120–121 }} 
* {{cite book | last=Serre | first=Jean-Pierre | authorlink=Jean-Pierre Serre | title=Local fields | others=Translated from the French by Marvin Jay Greenberg | series=Graduate Texts in Mathematics | volume=67 | publisher=[[Springer-Verlag]] | year=1979 | isbn=0-387-90424-7 | zbl=0423.12016 }}

[[Category:Algebraic number theory]]
[[Category:Abelian group theory]]