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Fix MolienSeries to not hang on certain invalid inputs #5919

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Merged
merged 2 commits into from
Jan 28, 2025
Merged

Fix MolienSeries to not hang on certain invalid inputs #5919

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c0793f8
Change MolienSeries to not hang on certain invalid inputs
fingolfin Jan 28, 2025
11bf72f
Update tst/testinstall/ctblmoli.tst
fingolfin Jan 28, 2025
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8 changes: 5 additions & 3 deletions lib/ctblmoli.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -208,6 +208,7 @@ InstallGlobalFunction( MolienSeries, function( arg )
chi, # character of `tbl', optional third argument
numers, # list of numerators of sum of polynomial quotients
denoms, # list of denominators of sum of polynomial quotients
R,
x, # indeterminate
tblclasses, # class lengths of `tbl'
orders, # representative orders of `tbl'
Expand Down Expand Up @@ -279,7 +280,8 @@ InstallGlobalFunction( MolienSeries, function( arg )
# `pol' is an additive polynomial.
numers:= [];
denoms:= [];
x:= Indeterminate( Rationals );
R:= UnivariatePolynomialRing(Rationals);
x:= R.1;
pol:= Zero( x );

tblclasses:= SizesConjugacyClasses( tbl );
Expand Down Expand Up @@ -361,7 +363,7 @@ InstallGlobalFunction( MolienSeries, function( arg )
# Split the summand into two summands, with denominators
# the special factor `f' resp. the remaining factors `F'.
f:= ( x ^ special[1] - 1 ) ^ special[2];
repr:= GcdRepresentation( F, f );
repr:= GcdRepresentation( R, F, f );

# Reduce the numerators if possible.
num:= numer * repr[1];
Expand Down Expand Up @@ -413,7 +415,7 @@ InstallGlobalFunction( MolienSeries, function( arg )
# $\prod_{j>i} f_j$ is stored in `F', and $f_i$ is in `f'.

# at first position $r_i$, at second position $q_i$
repr:= GcdRepresentation( F, f );
repr:= GcdRepresentation( R, F, f );

# The numerator $p_i$.
p:= q * repr[1];
Expand Down
11 changes: 11 additions & 0 deletions tst/testinstall/ctblmoli.tst
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Expand Up @@ -53,5 +53,16 @@ true
gap> ser = ser2;
true

#
# MolienSeries used to hang when given an non-character as input
# now an error is raised
#
gap> G:=Group((1,2,3,4,5,6,7),(5,6,7));;
gap> MolienSeries(NaturalCharacter(G));
( 1-z^3+z^6-z^9+z^12-z^15+z^18 ) / ( (1-z^7)*(1-z^5)*(1-z^4)*(1-z^3)^2*(1-z^2)\
*(1-z) )
gap> MolienSeries(-NaturalCharacter(G));
Error, <ns> must be a subset of <R>

#
gap> STOP_TEST("ctblmoli.tst");
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