Isoclinism of groups

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The notion of isoclinism seems to have been first introduced by Philip Hall mainly for the purpose of classifying finite p-groups, in his 1937 paper.

About this page

This page is mostly about the mappings that are used to define isoclinism. For more on the equivalence relation of being isoclinic, see isoclinic groups.


Short definition

An isoclinism is an isologism of groups with respect to the subvariety of abelian groups.

Full definition

For any group G, let \operatorname{Inn}(G) denote the inner automorphism group of G, G' denote the derived subgroup of G, and Z(G) denote the center of G.

Let \omega_G denote the map from \operatorname{Inn}(G) \times \operatorname{Inn}(G) to G' defined by first taking the map G \times G \to G' given as (x,y) \mapsto x^{-1}y^{-1}xy and then observing that the map is constant on the cosets of Z(G) \times Z(G).

An isoclinism of groups G_1 and G_2 is a pair (\zeta,\varphi) where \zeta is an isomorphism of \operatorname{Inn}(G_1) with \operatorname{Inn}(G_2) and \varphi is an isomorphism of G_1' with G_2' such that \varphi \circ \omega_{G_1} = \omega_{G_2} \circ (\zeta \times \zeta). Explicitly, this means that for any x,y \in \operatorname{Inn}(G_1), we have the following:

\varphi(\omega_{G_1}(x,y)) = \omega_{G_2}(\zeta(x),\zeta(y))

Pictorially, the following diagram must commute:

  \operatorname{Inn}(G_1) \times \operatorname{Inn}(G_1) & \stackrel{\zeta \times \zeta}{\to} & \operatorname{Inn}(G_2) \times \operatorname{Inn}(G_2) \\
  \downarrow^{\omega_{G_1}}  & & \downarrow^{\omega_{G_2}}\\
  G_1' & \stackrel{\varphi}{\to} & G_2'\\

Two groups are said to be isoclinic groups if there is an isoclinism between them.

Definition in terms of homoclinism

An isoclinism is an invertible homoclinism of groups, i.e., a homoclinism for which both the component homomorphisms are isomorphisms. Equivalently, it is an isomorphism in the category of groups with homoclinisms.


Journal references

Original use

  • The classification of prime-power groups by Philip Hall, Volume 69, (Year 1937): Official linkMore info: Definition introduced on Page 133 (Page 4 within the paper)

Other uses


Textbook references

Book Page number Chapter and section Contextual information View
Group Theory II (Grundlehren Der Mathematischen Wissenschaften 248) by Michio Suzuki. 10-digit ISBN 0387109161, 13-digit ISBN 978-0387109169More info 93 Chapter 4 (Finite p-groups), Definition 4.28 definition introduced explicitly, followed by facts about isoclinic groups