RAQ/lib/constructions.gi

## constructions.gi  RAQ  Implementations of other quandle constructions

## Inert objects with which to build trivial quandles.
DeclareCategory("IsLRInertum", IsMultiplicativeElement);
DeclareCategory("IsLInertum", IsLRInertum and IsLeftQuotientElement);
DeclareCategory("IsRInertum", IsLRInertum and IsRightQuotientElement);
DeclareCategoryCollections("IsLInertum");
DeclareCategoryCollections("IsRInertum");
LInertumFamily@ := NewFamily("LeftInertFamily", IsLInertum, 
                           IsLSelfDistElement and IsIdempotent);
SetMultiplicationTable(LInertumFamily@, []);
RInertumFamily@ := NewFamily("RightInertFamily", IsRInertum, 
                           IsRSelfDistElement and IsIdempotent);
SetMultiplicationTable(RInertumFamily@, []);
LInertumType@ := NewType(LInertumFamily@,
                         IsLInertum and IsPositionalObjectOneSlotRep);
RInertumType@ := NewType(RInertumFamily@,
                         IsRInertum and IsPositionalObjectOneSlotRep);
LInertum@ := i -> Objectify(LInertumType@, [i]);
RInertum@ := i -> Objectify(RInertumType@, [i]);

WhichI@ := obj -> obj![1];

InstallMethod(String, "for left inert objects", 
  [IsLInertum],
  obj -> Concatenation("LInertum@raq(", String(WhichI@(obj)), ")")
);
InstallMethod(String, "for rightt inert objects", 
  [IsRInertum],
  obj -> Concatenation("RInertum@raq(", String(WhichI@(obj)), ")")
);

InstallMethod(ViewString, "for left inert objects", 
  [IsLInertum],
  obj -> Concatenation("i", String(WhichI@(obj)), ">")
);
InstallMethod(ViewString, "for right inert objects", 
  [IsRInertum],
  obj -> Concatenation("<i", String(WhichI@(obj)))
);

InstallMethod(\=, "for two inert objects",
  IsIdenticalObj,
  [IsLRInertum, IsLRInertum],
  function(l,r) return l![1] = r![1]; end
);

InstallMethod(\<, "for two inert objects",
  IsIdenticalObj,
  [IsLRInertum, IsLRInertum],
  function(l,r) return l![1] < r![1]; end
);

InstallMethod(\*, "for a left inert object and anything",
  [IsLInertum, IsExtLElement],
  function(l,r) return r; end
);

InstallMethod(\*, "for anything and a right inert object",
  [IsExtRElement, IsRInertum], ReturnFirst
);

InstallOtherMethod(LeftQuotient, "for a left inert object and anything",
  [IsLInertum, IsExtLElement],
  function(l,r) return r; end
);

InstallOtherMethod(\/, "for anything and a right inert object",
  [IsExtRElement, IsRInertum], ReturnFirst
);

LeftTrivs@ := [];
RightTrivs@ := [];

InstallGlobalFunction(TrivialLeftQuandle,
  function(n)
    local trivs, I;
    if not IsBound(LeftTrivs@[n]) then
      trivs := List([1..n], LInertum@);
      I := LeftQuandleNC(CollectionsFamily(LInertumFamily@), trivs);
      SetIsTrivial(I, true);
      SetAsSSortedList(I, trivs);
      SetMultiplicationTable(I, ListWithIdenticalEntries(n, [1..n]));
      LeftTrivs@[n] := I;
    fi;
    return LeftTrivs@[n];
end);

InstallGlobalFunction(TrivialRightQuandle,
  function(n)
    local trivs, I;
    if not IsBound(RightTrivs@[n]) then
      trivs := List([1..n], RInertum@);
      I := RightQuandleNC(CollectionsFamily(RInertumFamily@), trivs);
      SetIsTrivial(I, true);
      SetAsSSortedList(I, trivs);
      SetMultiplicationTable(I,
        List([1..n], i -> ListWithIdenticalEntries(n, i)));
      RightTrivs@[n] := I;
    fi;
    return RightTrivs@[n];
end);

# The below needs to be immediate because otherwise it
# is pre-empted by the immediate method that all collections of size
# larger than one are non-trivial.
InstallImmediateMethod(IsTrivial, "for a left quasigroup",
  IsLeftQuasigroup and HasSize, 1,
  function(Q) 
    local gens;
    gens := GeneratorsOfLeftQuasigroup(Q);
    return ForAll(gens, x -> ForAll(gens, y -> x*y = y));
end);

InstallImmediateMethod(IsTrivial, "for a right quasigroup",
  IsRightQuasigroup and HasSize, 1,
  function(Q)
    local gens;
    gens := GeneratorsOfRightQuasigroup(Q);
    return ForAll(gens, x -> ForAll(gens, y -> y*x = y));
end);

## Convenience functions

InstallGlobalFunction(Quandle,
  function(arg)
    if Length(arg) = 0 then
      return LeftQuandleNC(CollectionsFamily(LInertumFamily@),[]);
    fi;
    if Length(arg) = 1 and IsInt(arg[1]) then
      return TrivialLeftQuandle(arg[1]);
    fi;
    ## Will put more stuff here, but for now, just a convenient wrapper for
    ## perms
    return LeftQuandleByPerms(arg);
end);

InstallGlobalFunction(Rack,
  function(arg)
    ## Will put more stuff here, but for now, just a convenient wrapper for
    ## perms
    return LeftRackByPerms(arg);
end);