MeatAxe  2.4
Programs for working with modular representations
precond - Precondensation of Tensor Products"

Command Line

precond Options Info M N 
Standard options, see Standard Command Line Options
Name of the tensor condensation data file.
Name of first module (left factor).
Name of second module (right factor).

Input Files

M.cfinfo, N.cfinfo
Constituent information.
MCf.std.1, MCf.std.2, ..., NCf.std.1, NCf.std.2, ...
Standard generators of the condensation subgroup H for each constituent.

Output Files

Tensor condensation info file.
Info.q.1, Info.q.2, ...
Embeddings for each constituent.
Info.p.1, Info.p.2, ...
Projections for each constituent.


This program performs two tasks:

  • It compares the irreducible constituents of MH and NH, and finds all pairs (Si,Tj) of constituents where Si≅Tj.
  • For each pair (S,T) of constituents found in step 1, the program calculates the embedding of (S⊗T)eH into S⊗T as an direct summand, and the corresponding projection of S⊗T onto (S⊗T)eH. If there is no peak word for a constituent, precond will issue a warning but continue. However, the P and Q matrices for this constituent are zero.

Implementation Details

Step 1, matching of constituents, is implemented in the same way as in chop and cfcomp, i.e., by using the standard basis with respect to identifying words. Step 2 is based on two observations:

  • (A): V⊗V*≅Homk(V,V)$, and (S⊗T)e_H≅EndkH(V) as $kH$-Modules.
  • (B): There is a natural, H-invariant non-degenerate scalar product on Homk(V,V), given by Γ(φ,ψ)=Trace(φ∘ψ).

From (A) it is clear that calculating the embedding of (S⊗T)eH into S⊗T is equivalent to computing a basis of EndkH(V). The latter is easily accomplished using the peak word of V. As a consequence of the second observation, there is a natural one-to-one correspondence between H-invariant linear forms on Hom_k(V,V) and EndkH(V), which is used to calculate the projection from Homk(V,V) on EndkH(V).

More details on the algorithm used in Step 2 can be found in [Ri98].

MeatAxe 2.4 documentation, generated on Wed Jan 7 2015 08:38:36