// p5g3q8mo.m
print "\nConstruct a 3-generator 5-group anti-Hughes group of class 9",
"\nwhich satisfies 8 commutator defining relators and in which",
"\nthe normal closures of two generators have class 4",
"\nwhile the normal closure of the other generator, c, is abelian.",
"\nThen repeatedly factor out complements to c^5 in the centre",
"\nof the group, to obtain smaller counterexamples";
F := FreeGroup(3); p := 5;
Q := quo < F | (c,a,b), (c,b,b,b,a), (b,a,a,a,c,a), (b,a,b,b,c,b),
(b,a,a,a,c,b,b), (b,a,a,b,b,b,c),
(b,a,a,a,b,a,b,c), (b,a,a,a,b,b,a,b) >;
P := pQuotientProcess( Q, p, 2 : Exponent := p );
NextClass( ~P : Exponent := p, MaxOccurrence := [4,4,1] );
NextClass( ~P : Exponent := p, MaxOccurrence := [4,4,1] );
NextClass( ~P : Exponent := p, MaxOccurrence := [4,4,1] );
NextClass( ~P : Exponent := p, MaxOccurrence := [4,4,1] );
NextClass( ~P : Exponent := p, MaxOccurrence := [4,4,1] );
NextClass( ~P : Exponent := p, MaxOccurrence := [4,4,1] );
printf "\nThe class 8 quotient of the group has order %o^%o\n",
p, FactoredOrder(P)[1][2];
// Note: this implementation of the p-quotient algorithm uses
// the lower exponent-$p$-central series so we must allow
// 2 (not 1) occurrences of c in the covering group
// to allow the order of c to become 25 in the cover
NextClass( ~P : Exponent := 0, MaxOccurrence := [4,4,2] );
G := ExtractGroup(P);
printf "\nThe p-covering group G has order %o^%o and class %o",
p, FactoredOrder(G)[1][2], pClass(G);
print "\nG is generated by a, b and c; and c has order", Order(c),
"\n[b,a] has order", Order( (b,a) ), "; [c,a] has order", Order( (c,a) ),
"; [c,b] has order", Order( (c,b) ),
"\ngamma_2(G) is the normal closure of < [b,a], [c,a], [c,b] >,",
"\nand gamma_2(G) has class at most 4, so gamma_2(G) has exponent 5";
print "Also factor out the derived group of the normal closure of c",
"\n(This became non-trivial in the p-cover computation)";
C := ncl< G | c >; C1 := DerivedGroup(C); G := quo< G | C1 >;
printf "\nThe group now has order %o^%o", p, FactoredOrder(G)[1][2];
print "\nNow compute suitable 5th powers of elements outside G'";
load twB35c5qmo441; // should be already computed by gettestwq.m
S := [ x^p : x in testwords ];
H := quo< G | S >;
printf "The quotient group, H, has order %o^%o", p, FactoredOrder(H)[1][2];
print "\nH is generated by a, b and c; and c^5 =", c^p,
"\nH is anti-Hughes; the normal closures of a and b both have class 4",
"\nwhile the normal closure of c is abelian",
"\nand this implies that gamma_3(G) has class 3",
"\nso every element cg, g in gamma_3, has order 25",
"\nWe also have:",
"c[b,a] has order", Order(c*(b,a)), "; c[c,a] has order",
Order(c*(c,a)), "; c[c,b] has order", Order(c*(c,b)),
"\nSo the Hughes subgroup is G' and c^5 generates gamma_9(H)";
CurrentQ := H;
//Repeatedly factor out complements for c^5 in the centre
Z := Centre(CurrentQ);
while Order(Z) ne p do
rank := FactoredOrder(Z)[1][2];
printf "Z, its centre, has order %o^%o", p, rank;
print "\nNow build a complement for c^5 in Z\n";
ZGens := [ CurrentQ!Z.i : i in [1..rank] ];
_, index := Max( Eltseq( c^p ) );
ComplGens := [ ZGens[i] * (CurrentQ.index)^-Eltseq(ZGens[i])[index]
: i in [1..rank] ];
NextQ := quo< CurrentQ | ComplGens >;
printf "Factor it out to get an anti-Hughes group of order %o^%o",
p, FactoredOrder(NextQ)[1][2];
print "\ngenerated by a, b and c; and c^5 =", c^p;
CurrentQ := NextQ;
Z := Centre(CurrentQ);
end while;
printf "The centre is generated by %o and has order %o^%o",
CurrentQ!(Z.1), p, FactoredOrder(Z)[1][2];
printf "\nSo this method reduces to an anti-Hughes group with order %o^%o",
p, FactoredOrder(CurrentQ)[1][2];
print "\nwhich is as far as we can reduce the group (by this method!)";