percentage of homology

[Michel PETITJEAN]

Where are the various definitions of the percentage of homology
for multiple sequence alignments ?
I am looking for the references rahter than the definitions themselves.
Thanks.

Michel Petitjean,                     Email: petitjean at itodys.jussieu.fr
ITODYS (CNRS, URA34)                         ptitjean at ccr.jussieu.fr