<Section>
                <Heading>
                    Numerical semigroups with the Arf property and Arf closures
                </Heading>
                Numerical semigroups with the Arf property are a special kind of numerical semigroups with 
                maximal embedding dimension. A numerical semigroup <M>S</M> is Arf if for every <M>x,y,z</M> in
                <M>S</M> with <M>x\geq y\geq z</M>, one has that <M>x+y-z\in S</M>.
                
                <P/> 
                The intersection of two Arf numerical semigroups is again Arf, and thus we can consider the 
                Arf closure of a set of nonnegative integers with greatest common divisor equal to one. Analogously
                as with MED numerical semigroups, we define Arf systems of generators and minimal Arf generating
                system for an Arf numerical semigroup. These are also unique(see <Cite Key="RGGB04"></Cite>).
                
                
                <ManSection>
                    <Func Arg="S" Name="IsArfNumericalSemigroup"></Func>
                    <Description>
                        <A>S</A> is a numerical semigroup.    
                        <P/>
                        Returns true if <A>S</A> is an Arf numerical semigroup and false otherwise. 
                        <Example><![CDATA[
gap>  IsArfNumericalSemigroup(NumericalSemigroup(3,5,7)); 
true 
gap>  IsArfNumericalSemigroup(NumericalSemigroup(3,7,11)); 
false 
gap> IsMEDNumericalSemigroup(NumericalSemigroup(3,7,11)); 
true
]]>
                        </Example>          
                    </Description>
                </ManSection>
                
                <ManSection>
                    <Func Arg="S" Name="ArfNumericalSemigroupClosure"></Func>
                    <Description>
                        <A>S</A> is a numerical semigroup.    
                        <P/>
                        Returns the Arf closure of <A>S</A>.
                        <Example><![CDATA[
gap> ArfNumericalSemigroupClosure(NumericalSemigroup(3,7,11)); 
<Numerical semigroup> 
gap> MinimalGeneratingSystemOfNumericalSemigroup(last); 
[ 3, 7, 8 ]
]]>
                        </Example>          
                    </Description>
                </ManSection>
                
                <ManSection>
                    <Func Arg="S" Name="MinimalArfGeneratingSystemOfArfNumericalSemigroup"></Func>
                    <Description>
                        <A>S</A> is an Arf numerical semigroup.    
                        <P/>
                        Returns the minimal MED generating system of <A>S</A>.
                        <Example><![CDATA[
gap> MinimalArfGeneratingSystemOfArfNumericalSemigroup( 
> NumericalSemigroup(3,7,8)); 
[ 3, 7 ]
]]>
                        </Example>          
                    </Description>
                </ManSection>
                
                
                
            </Section>