Mod-5-Cohomology of J2, a group of order 604800

About the group Ring generators Ring relations Completion information Restriction maps

General information on the group

  • J2 is a group of order 604800.
  • The group order factors as 27 · 33 · 52 · 7.
  • The group is defined by Group([(1,84)(2,20)(3,48)(4,56)(5,82)(6,67)(7,55)(8,41)(9,35)(10,40)(11,78)(12,100)(13,49)(14,37)(15,94)(16,76)(17,19)(18,44)(21,34)(22,85)(23,92)(24,57)(25,75)(26,28)(27,64)(29,90)(30,97)(31,38)(32,68)(33,69)(36,53)(39,61)(42,73)(43,91)(45,86)(46,81)(47,89)(50,93)(51,96)(52,72)(54,74)(58,99)(59,95)(60,63)(62,83)(65,70)(66,88)(71,87)(77,98)(79,80),(1,80,22)(2,9,11)(3,53,87)(4,23,78)(5,51,18)(6,37,24)(8,27,60)(10,62,47)(12,65,31)(13,64,19)(14,61,52)(15,98,25)(16,73,32)(17,39,33)(20,97,58)(21,96,67)(26,93,99)(28,57,35)(29,71,55)(30,69,45)(34,86,82)(38,59,94)(40,43,91)(42,68,44)(46,85,89)(48,76,90)(49,92,77)(50,66,88)(54,95,56)(63,74,72)(70,81,75)(79,100,83)]).
  • It is non-abelian.
  • It has 5-Rank 2.
  • The centre of a Sylow 5-subgroup has rank 2.
  • Its Sylow 5-subgroup has a unique conjugacy class of maximal elementary abelian subgroups, which is of rank 2.

Structure of the cohomology ring

This cohomology ring is isomorphic to the cohomology ring of a subgroup, namely H*(SmallGroup(300,25); GF(5)).

General information

  • The cohomology ring is of dimension 2 and depth 2.
  • The depth coincides with the Duflot bound.
  • The Poincaré series is
    1  −  t  +  t2  −  t3  +  t4  −  t5  +  t6  −  t7  +  t8  −  t9  +  t10
    ( − 1  +  t)2 · (1  +  t  +  t2) · (1  +  t2)2 · (1  −  t2  +  t4)
  • The a-invariants are -∞,-∞,-2. They were obtained using the filter regular HSOP of the Hilbert-Poincaré test.
  • The filter degree type of any filter regular HSOP is [-1, -2, -2].

About the group Ring generators Ring relations Completion information Restriction maps

Ring generators

The cohomology ring has 4 minimal generators of maximal degree 12:

  1. a_3_0, a nilpotent element of degree 3
  2. c_4_0, a Duflot element of degree 4
  3. a_11_1, a nilpotent element of degree 11
  4. c_12_1, a Duflot element of degree 12

About the group Ring generators Ring relations Completion information Restriction maps

Ring relations

There are 2 "obvious" relations:
   a_3_02, a_11_12

Apart from that, there are no relations.

About the group Ring generators Ring relations Completion information Restriction maps

Data used for the Hilbert-Poincaré test

  • We proved completion in degree 14 using the Hilbert-Poincaré criterion.
  • However, the last relation was already found in degree 0 and the last generator in degree 12.
  • The following is a filter regular homogeneous system of parameters:
    1. c_4_0, an element of degree 4
    2. c_12_1, an element of degree 12
  • The above filter regular HSOP forms a Duflot regular sequence.
  • The Raw Filter Degree Type of the filter regular HSOP is [-1, -1, 14].

About the group Ring generators Ring relations Completion information Restriction maps

Restriction maps

Expressing the generators as elements of H*(SmallGroup(300,25); GF(5))

  1. a_3_0 → a_3_0
  2. c_4_0 → c_4_0
  3. a_11_1 → a_11_1
  4. c_12_1 → c_12_1

Restriction map to the greatest el. ab. subgp. in the centre of a Sylow subgroup, which is of rank 2

  1. a_3_0 → c_2_2·a_1_1 − c_2_2·a_1_0 − c_2_1·a_1_1 − c_2_1·a_1_0, an element of degree 3
  2. c_4_0 → c_2_22 − 2·c_2_1·c_2_2 − c_2_12, an element of degree 4
  3. a_11_1 → c_2_1·c_2_24·a_1_0 + 2·c_2_12·c_2_23·a_1_1 − c_2_12·c_2_23·a_1_0
       − c_2_13·c_2_22·a_1_1 − 2·c_2_13·c_2_22·a_1_0 − c_2_14·c_2_2·a_1_1, an element of degree 11
  4. c_12_1 → c_2_12·c_2_24 + c_2_13·c_2_23 − c_2_14·c_2_22, an element of degree 12

About the group Ring generators Ring relations Completion information Restriction maps


Simon King
Department of Mathematics and Computer Science
Friedrich-Schiller-Universität Jena
07737 Jena
GERMANY
E-mail: simon dot king at uni hyphen jena dot de
Tel: +49 (0)3641 9-46161
Fax: +49 (0)3641 9-46162
Office: Zi. 3529, Ernst-Abbe-Platz 2

Last change: 14.12.2010