Cohomology of group number 25 of order 32

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General information on the group

  • The group is also known as D8xC4, the Direct product D8 x C_4.
  • The group has 3 minimal generators and exponent 4.
  • It is non-abelian.
  • It has p-Rank 3.
  • Its center has rank 2.
  • It has 2 conjugacy classes of maximal elementary abelian subgroups, which are all of rank 3.

Structure of the cohomology ring

General information

  • The cohomology ring is of dimension 3 and depth 3.
  • The depth exceeds the Duflot bound, which is 2.
  • The Poincaré series is
     − 1
    (t  −  1)3
  • The a-invariants are -∞,-∞,-∞,-3. They were obtained using the filter regular HSOP of the Benson test.

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Ring generators

The cohomology ring has 5 minimal generators of maximal degree 2:

  1. a_1_0, a nilpotent element of degree 1
  2. b_1_1, an element of degree 1
  3. b_1_2, an element of degree 1
  4. c_2_4, a Duflot regular element of degree 2
  5. c_2_5, a Duflot regular element of degree 2

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Ring relations

There are 2 minimal relations of maximal degree 2:

  1. a_1_02
  2. b_1_1·b_1_2 + a_1_0·b_1_1

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Data used for Benson′s test

  • Benson′s completion test succeeded in degree 3.
  • However, the last relation was already found in degree 2 and the last generator in degree 2.
  • The following is a filter regular homogeneous system of parameters:
    1. c_2_4, a Duflot regular element of degree 2
    2. c_2_5, a Duflot regular element of degree 2
    3. b_1_2 + b_1_1, an element of degree 1
  • The Raw Filter Degree Type of that HSOP is [-1, -1, -1, 2].
  • The filter degree type of any filter regular HSOP is [-1, -2, -3, -3].

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Restriction maps

Restriction map to the greatest central el. ab. subgp., which is of rank 2

  1. a_1_0 → 0, an element of degree 1
  2. b_1_1 → 0, an element of degree 1
  3. b_1_2 → 0, an element of degree 1
  4. c_2_4 → c_1_02, an element of degree 2
  5. c_2_5 → c_1_12, an element of degree 2

Restriction map to a maximal el. ab. subgp. of rank 3

  1. a_1_0 → 0, an element of degree 1
  2. b_1_1 → c_1_2, an element of degree 1
  3. b_1_2 → 0, an element of degree 1
  4. c_2_4 → c_1_0·c_1_2 + c_1_02, an element of degree 2
  5. c_2_5 → c_1_12, an element of degree 2

Restriction map to a maximal el. ab. subgp. of rank 3

  1. a_1_0 → 0, an element of degree 1
  2. b_1_1 → 0, an element of degree 1
  3. b_1_2 → c_1_2, an element of degree 1
  4. c_2_4 → c_1_0·c_1_2 + c_1_02, an element of degree 2
  5. c_2_5 → c_1_22 + c_1_12, an element of degree 2

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Simon A. King David J. Green
Fakultät für Mathematik und Informatik Fakultät für Mathematik und Informatik
Friedrich-Schiller-Universität Jena Friedrich-Schiller-Universität Jena
Ernst-Abbe-Platz 2 Ernst-Abbe-Platz 2
D-07743 Jena D-07743 Jena
Germany Germany
E-mail: simon dot king at uni hyphen jena dot de
Tel: +49 (0)3641 9-46184
Fax: +49 (0)3641 9-46162
Office: Zi. 3524, Ernst-Abbe-Platz 2
E-mail: david dot green at uni hyphen jena dot de
Tel: +49 3641 9-46166
Fax: +49 3641 9-46162
Office: Zi 3512, Ernst-Abbe-Platz 2

Last change: 25.08.2009