<Chapter Label="ch:Z_pi">
<Heading>Semilocalizations of the Integers</Heading>
<Ignore Remark="set screen width to 75, for the example tester">
<Example>
<![CDATA[
gap> SizeScreen([75,24]);;
]]>
</Example>
</Ignore>
This package implements residue class unions of the semilocalizations
<M>&ZZ;_{(\pi)}</M> of the ring of integers. It also provides the
underlying &GAP; implementation of these rings themselves.
<Section Label="sec:DefiningZ_pi">
<Heading>Entering semilocalizations of the integers</Heading>
<ManSection>
<Func Name="Z_pi" Arg="pi" Label="by set of non-invertible primes"/>
<Func Name="Z_pi" Arg="p" Label="by non-invertible prime"/>
<Returns>
the ring <M>&ZZ;_{(\pi)}</M> or the ring <M>&ZZ;_{(p)}</M>,
respectively.
</Returns>
<Description>
The returned ring has the property <C>IsZ&uscore;pi</C>.
The set <A>pi</A> of non-invertible primes can be retrieved
by the operation <C>NoninvertiblePrimes</C>.
<Index Key="IsZ_pi"><C>IsZ&uscore;pi</C></Index>
<Index Key="NoninvertiblePrimes" Subkey="of a semilocalization of Z">
<C>NoninvertiblePrimes</C>
</Index>
<Example>
<![CDATA[
gap> R := Z_pi(2);
Z_( 2 )
gap> S := Z_pi([2,5,7]);
Z_( 2, 5, 7 )
]]>
</Example>
</Description>
</ManSection>
<Section Label="sec:MethodsForZ_pi">
<Heading>Methods for semilocalizations of the integers</Heading>
<Index Key="StandardAssociate"
Subkey="of an element of a semilocalization of Z">
<C>StandardAssociate</C>
</Index>
<Index Key="Gcd" Subkey="of elements of a semilocalization of Z">
<C>Gcd</C>
</Index>
<Index Key="Lcm" Subkey="of elements of a semilocalization of Z">
<C>Lcm</C>
</Index>
<Index Key="Factors" Subkey="of an element of a semilocalization of Z">
<C>Factors</C>
</Index>
<Index Key="IsUnit" Subkey="for an element of a semilocalization of Z">
<C>IsUnit</C>
</Index>
There are methods for the operations <C>in</C>, <C>Intersection</C>,
<C>IsSubset</C>, <C>StandardAssociate</C>, <C>Gcd</C>, <C>Lcm</C>,
<C>Factors</C> and <C>IsUnit</C> available for semilocalizations of the
integers. For the documentation of these operations, see the &GAP;
reference manual. The standard associate of an element of a ring
<M>&ZZ;_{(\pi)}</M> is defined by the product of the non-invertible
prime factors of its numerator.
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