4.15 Centralizer can fail if matrix has denominators

[Genya-Z]

This fails in 4.15

G := Group([[[0, 1, 0, 0], [1, 0, 0, 0], [0, 0, 1, 0], [0, 0, 0, 1]],
 [[1, 0, 0, 0], [0, 0, 1, 0], [0, -1, 0, 0], [0, 0, 0, 1]],
 [[1, 0, 0, 0], [0, 1, 0, 0], [0, 0, 0, 1], [0, 0, 1, 0]]]);

Centralizer(G,[[0, 0, 0, 0], [0, 1/3, 1/3, 1/3], [0, 1/3, 1/3, 1/3], [0, 1/3, 1/3, 1/3]]);

session:

 
 ┌───────┐   GAP 4.15.0-dirty
 │  GAP  │   https://www.gap-system.org
 └───────┘   Architecture: x86_64-pc-linux-gnu-default64-kv10
 Configuration:  gmp 6.3.0, GASMAN, readline
 Loading the library and packages ...
#I  packagemanager package is not available. Check that the name is correct
#I  and it is present in one of the GAP root directories (see '??RootPaths')
 Packages:   Alnuth 3.2.1, AtlasRep 2.1.9, AutPGrp 1.11.1, CRISP 1.4.8, CTblLib 1.3.11, FactInt 1.6.3, 
             FGA 1.5.0, GAPDoc 1.6.7, IRREDSOL 1.4.4, LAGUNA 3.9.7, Polenta 1.3.11, Polycyclic 2.17, 
             PrimGrp 4.0.1, RadiRoot 2.9, ResClasses 4.7.4, SmallGrp 1.5.4, Sophus 1.27, TomLib 1.2.11, 
             TransGrp 3.6.5, utils 0.92
 Try '??help' for help. See also '?copyright', '?cite' and '?authors'
gap> G := Group([[[0, 1, 0, 0], [1, 0, 0, 0], [0, 0, 1, 0], [0, 0, 0, 1]],
>  [[1, 0, 0, 0], [0, 0, 1, 0], [0, -1, 0, 0], [0, 0, 0, 1]],
>  [[1, 0, 0, 0], [0, 1, 0, 0], [0, 0, 0, 1], [0, 0, 1, 0]]]);
<matrix group with 3 generators>
gap> 
gap> Centralizer(G,[[0, 0, 0, 0], [0, 1/3, 1/3, 1/3], [0, 1/3, 1/3, 1/3], [0, 1/3, 1/3, 1/3]]);
Error, ModRat: for <r>/<s> mod <n>, <s>/gcd(<r>,<s>) and <n> must be coprime in
  rat mod Characteristic( ffe ) at /usr/share/gap/lib/ffe.gi:90 called from 
x * e at /usr/share/gap/lib/grpramat.gi:330 called from
map!.fun( elm ) at /usr/share/gap/lib/mapprep.gi:616 called from
ImagesRepresentative( nice, obj2 ) at /usr/share/gap/lib/grpnice.gd:637 called from
CallFuncList( meth, obj ) at /usr/share/gap/lib/grpnice.gd:264 called from
oper( super, sub ) at /usr/share/gap/lib/domain.gd:452 called from
...  at *stdin*:5
type 'quit;' to quit to outer loop

[fingolfin]

Thanks for your report, much appreciated. I'll provide a fix for this shortly, and it will be addressed in GAP 4.15.1