Mod-3-Cohomology of MathieuGroup(9), a group of order 72

About the group Ring generators Ring relations Completion information Restriction maps


General information on the group

  • MathieuGroup(9) is a group of order 72.
  • The group order factors as 23 · 32.
  • The group is defined by Group([(1,4,9,8)(2,5,3,6),(1,6,5,2)(3,7,9,8)]).
  • It is non-abelian.
  • It has 3-Rank 2.
  • The centre of a Sylow 3-subgroup has rank 2.
  • Its Sylow 3-subgroup has a unique conjugacy class of maximal elementary abelian subgroups, which is of rank 2.


Structure of the cohomology ring

The computation was based on 7 stability conditions for H*(SmallGroup(9,2); GF(3)).

General information

  • The cohomology ring is of dimension 2 and depth 2.
  • The depth coincides with the Duflot bound.
  • The Poincaré series is
    1  −  2·t  +  4·t2  −  5·t3  +  5·t4  −  5·t5  +  4·t6  −  2·t7  +  t8

    ( − 1  +  t)2 · (1  +  t2)2 · (1  +  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 8 minimal generators of maximal degree 12:

  1. a_2_0, a nilpotent element of degree 2
  2. a_3_0, a nilpotent element of degree 3
  3. a_7_1, a nilpotent element of degree 7
  4. a_7_0, a nilpotent element of degree 7
  5. c_8_1, a Duflot element of degree 8
  6. c_8_0, a Duflot element of degree 8
  7. a_11_0, a nilpotent element of degree 11
  8. c_12_0, a Duflot element of degree 12

About the group Ring generators Ring relations Completion information Restriction maps

Ring relations

There are 4 "obvious" relations:
   a_3_02, a_7_02, a_7_12, a_11_02

Apart from that, there are 16 minimal relations of maximal degree 24:

  1. a_2_02
  2. a_2_0·a_3_0
  3. a_2_0·a_7_0
  4. a_2_0·a_7_1
  5. a_3_0·a_7_0 − a_2_0·c_8_0
  6. a_3_0·a_7_1 − a_2_0·c_8_1
  7. a_2_0·a_11_0
  8. a_3_0·a_11_0 − a_2_0·c_12_0
  9. a_7_0·a_7_1 + a_2_0·c_12_0
  10. c_12_0·a_3_0 + c_8_1·a_7_0 − c_8_0·a_7_1
  11. a_7_0·a_11_0 − a_2_0·c_8_0·c_8_1
  12. a_7_1·a_11_0 − a_2_0·c_8_02
  13. c_12_0·a_7_0 − c_8_0·a_11_0 − c_8_0·c_8_1·a_3_0
  14. c_12_0·a_7_1 − c_8_1·a_11_0 − c_8_02·a_3_0
  15. c_12_0·a_11_0 + c_8_0·c_8_1·a_7_1 − c_8_02·a_7_0
  16. c_12_02 + c_8_0·c_8_12 − c_8_03


About the group Ring generators Ring relations Completion information Restriction maps

Data used for the Hilbert-Poincaré test

  • We proved completion in degree 24 using the Hilbert-Poincaré criterion.
  • The completion test was perfect: It applied in the last degree in which a generator or relation was found.
  • The following is a filter regular homogeneous system of parameters:
    1. c_8_1, an element of degree 8
    2. c_8_0, an element of degree 8
  • 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(9,2); GF(3))

  1. a_2_0a_1_0·a_1_1
  2. a_3_0c_2_2·a_1_0 − c_2_1·a_1_1
  3. a_7_1c_2_23·a_1_0 − c_2_13·a_1_1
  4. a_7_0c_2_23·a_1_1 + c_2_1·c_2_22·a_1_0 + c_2_12·c_2_2·a_1_1 + c_2_13·a_1_0
  5. c_8_1c_2_1·c_2_23 − c_2_13·c_2_2
  6. c_8_0c_2_24 − c_2_12·c_2_22 + c_2_14
  7. a_11_0c_2_25·a_1_1 + c_2_12·c_2_23·a_1_1 + c_2_13·c_2_22·a_1_0 + c_2_15·a_1_0
  8. c_12_0c_2_26 + c_2_12·c_2_24 + c_2_14·c_2_22 + c_2_16

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

  1. a_2_0a_1_0·a_1_1, an element of degree 2
  2. a_3_0c_2_2·a_1_0 − c_2_1·a_1_1, an element of degree 3
  3. a_7_1c_2_23·a_1_0 − c_2_13·a_1_1, an element of degree 7
  4. a_7_0c_2_23·a_1_1 + c_2_1·c_2_22·a_1_0 + c_2_12·c_2_2·a_1_1 + c_2_13·a_1_0, an element of degree 7
  5. c_8_1c_2_1·c_2_23 − c_2_13·c_2_2, an element of degree 8
  6. c_8_0c_2_24 − c_2_12·c_2_22 + c_2_14, an element of degree 8
  7. a_11_0c_2_25·a_1_1 + c_2_12·c_2_23·a_1_1 + c_2_13·c_2_22·a_1_0 + c_2_15·a_1_0, an element of degree 11
  8. c_12_0c_2_26 + c_2_12·c_2_24 + c_2_14·c_2_22 + c_2_16, 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