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Fix a case where Coefficients(B,v) returned a list instead of fail although the vector v did not lie in the underlying vector space V of B #5335

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Jan 19, 2023
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Fix a case where Coefficients(B,v) returned a list instead of fail although the vector v did not lie in the underlying vector space V of B #5335

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8 changes: 5 additions & 3 deletions lib/vspcrow.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -336,6 +336,7 @@ COEFFS_SEMI_ECH_BASIS:=function( B, v )
local vectors, # basis vectors of `B'
heads, # heads info of `B'
len, # length of `v'
F, # allowed coefficients
coeff, # coefficients list, result
i, # loop over `v'
pos; # heads position
Expand All @@ -351,6 +352,7 @@ COEFFS_SEMI_ECH_BASIS:=function( B, v )
if len <> Length( heads ) then
return fail;
fi;
F:= LeftActingDomain( UnderlyingLeftModule( B ) );

# Preset the coefficients list with zeroes.
coeff:= ListWithIdenticalEntries( Length( vectors ), Zero( v[1] ) );
Expand All @@ -360,11 +362,11 @@ COEFFS_SEMI_ECH_BASIS:=function( B, v )
i:= PositionNonZero( v );
while i <= len do
pos:= heads[i];
if pos <> 0 then
if pos = 0 or not v[i] in F then
return fail;
else
coeff[ pos ]:= v[i];
AddRowVector( v, vectors[ pos ], - v[i] );
else
return fail;
fi;
i:= PositionNonZero( v );
od;
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6 changes: 6 additions & 0 deletions tst/testinstall/vspcrow.tst
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Expand Up @@ -91,6 +91,12 @@ gap> BaseDomain( b[1] );
GF(2^2)
gap> b[1] = vecs[1];
true
gap> v:= GF(2)^1;;
gap> b:= Basis( v, [ [ Z(2) ] ] );;
gap> Coefficients( b, [ Z(4) ] );
fail
gap> SiftedVector( b, [ Z(4) ] );
fail

#############################################################################
##
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