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\newcommand{\Axiom}{{\tt Axiom}}
\begin{document}
\title{dirichlet.spad}
\author{Martin Rubey}
\maketitle
\begin{abstract}
  The domain defined in this file models the Dirichlet ring,
\end{abstract}
\tableofcontents

\section{The Dirichlet Ring}
The Dirichlet Ring is the ring of arithmetical functions
$$ f: \mathbb N_+ \rightarrow R$$
(see \url{http://en.wikipedia.org/wiki/Arithmetic_function}) together
with the Dirichlet convolution (see
\url{http://en.wikipedia.org/wiki/Dirichlet_convolution}) as
multiplication and component-wise addition.  Since we can consider
the values an arithmetic functions assumes as the coefficients of a
Dirichlet generating series, we call $R$ the coefficient ring of a
function.

In general we only assume that the coefficient ring $R$ is a ring.
If $R$ happens to be commutative, then so is the Dirichlet ring, and
in this case it is even an algebra.

Apart from the operations inherited from those categories, we only
provide some convenient coercion functions.


\section{domain DIRRING DirichletRing}
\nwfilename{dirichlet.spad.pamphlet}\nwbegincode{1}\moddef{domain DIRRING DirichletRing}\endmoddef\nwstartdeflinemarkup\nwenddeflinemarkup
)abbrev domain DIRRING DirichletRing
++ Author: Martin Rubey
++ Description: DirichletRing is the ring of arithmetical functions
++ with Dirichlet convolution as multiplication
DirichletRing(Coef: Ring):
            Exports == Implementation where

    PI ==> PositiveInteger
    FUN ==> PI -> Coef

    Exports ==> Join(Ring, Eltable(PI, Coef)) with

        if Coef has CommutativeRing then 
            IntegralDomain

        if Coef has CommutativeRing then 
            Algebra Coef

        coerce: FUN -> %
        coerce: % -> FUN
        coerce: Stream Coef -> %
        coerce: % -> Stream Coef

        zeta: constant -> %
        ++ zeta() returns the function which is constantly one

        multiplicative?: % -> Boolean
        ++ multiplicative?(a) returns true if the first
        ++ $streamCount$Lisp coefficients of a are multiplicative 

        additive?: % -> Boolean
        ++ additive?(a) returns true if the first
        ++ $streamCount$Lisp coefficients of a are additive 

    Implementation ==> add

        Rep := Record(function: FUN)

        per(f: Rep): % == f pretend %   
        rep(a: %): Rep == a pretend Rep 

        elt(a: %, n: PI): Coef ==
            f: FUN := (rep a).function
            f n

        coerce(a: %): FUN == (rep a).function

        coerce(f: FUN): % == per [f]

        indices: Stream Integer 
                := integers(1)$StreamTaylorSeriesOperations(Integer)

        coerce(a: %): Stream Coef ==
            f: FUN := (rep a).function
            map((n: Integer): Coef +-> f(n::PI), indices)
               $StreamFunctions2(Integer, Coef)

        coerce(f: Stream Coef): % == 
            ((n: PI): Coef +-> f.(n::Integer))::%

        coerce(f: %): OutputForm == f::Stream Coef::OutputForm

        1: % == 
            ((n: PI): Coef +-> (if one? n then 1$Coef else 0$Coef))::%

        0: % == 
            ((n: PI): Coef +-> 0$Coef)::%

        zeta: % ==
            ((n: PI): Coef +-> 1$Coef)::%

        (f: %) + (g: %) == 
            ((n: PI): Coef +-> f(n)+g(n))::%

        - (f: %) ==
            ((n: PI): Coef +-> -f(n))::%

        (a: Integer) * (f: %) ==
            ((n: PI): Coef +-> a*f(n))::%

        (a: Coef) * (f: %) ==
            ((n: PI): Coef +-> a*f(n))::%

        import IntegerNumberTheoryFunctions

        (f: %) * (g: %) == 
            conv := (n: PI): Coef +-> _
                 reduce((a: Coef, b: Coef): Coef +-> a + b, _
                        [f(d::PI) * g((n quo d)::PI) for d in divisors(n::Integer)], 0)
                        $ListFunctions2(Coef, Coef)
            conv::%

        unit?(a: %): Boolean == not (recip(a(1$PI))$Coef case "failed")

        qrecip: (%, Coef, PI) -> Coef
        qrecip(f: %, f1inv: Coef, n: PI): Coef ==
            if one? n then f1inv
            else 
                -f1inv * reduce(_+, [f(d::PI) * qrecip(f, f1inv, (n quo d)::PI) _
                                     for d in rest divisors(n)], 0) _
                               $ListFunctions2(Coef, Coef)

        recip f ==
            if (f1inv := recip(f(1$PI))$Coef) case "failed" then "failed"
            else 
                mp := (n: PI): Coef +-> qrecip(f, f1inv, n)

                mp::%::Union(%, "failed")

        multiplicative? a ==
            n: Integer := _$streamCount$Lisp
            for i in 2..n repeat
                fl := factors(factor i)$Factored(Integer)
                rl := [a.(((f.factor)::PI)^((f.exponent)::PI)) for f in fl]
                if a.(i::PI) ~= reduce((r:Coef, s:Coef):Coef +-> r*s, rl)
                then 
                    output(i::OutputForm)$OutputPackage
                    output(rl::OutputForm)$OutputPackage
                    return false
            true

        additive? a ==
            n: Integer := _$streamCount$Lisp
            for i in 2..n repeat
                fl := factors(factor i)$Factored(Integer)
                rl := [a.(((f.factor)::PI)^((f.exponent)::PI)) for f in fl]
                if a.(i::PI) ~= reduce((r:Coef, s:Coef):Coef +-> r+s, rl)
                then 
                    output(i::OutputForm)$OutputPackage
                    output(rl::OutputForm)$OutputPackage
                    return false
            true


\nwendcode{}\nwbegindocs{2}\nwdocspar
\section{License}
\nwenddocs{}\nwbegincode{3}\moddef{license}\endmoddef\nwstartdeflinemarkup\nwenddeflinemarkup
--Copyright (c) 2010, Martin Rubey <Martin.Rubey@math.uni-hannover.de>
--
--Redistribution and use in source and binary forms, with or without
--modification, are permitted provided that the following conditions are
--met:
--
--    - Redistributions of source code must retain the above copyright
--      notice, this list of conditions and the following disclaimer.
--
--    - Redistributions in binary form must reproduce the above copyright
--      notice, this list of conditions and the following disclaimer in
--      the documentation and/or other materials provided with the
--      distribution.
--
--THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
--IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
--TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
--PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
--OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
--EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
--PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
--PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
--LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
--NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
--SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
\nwendcode{}\nwbegindocs{4}\nwdocspar

\nwenddocs{}\nwbegincode{5}\moddef{*}\endmoddef\nwstartdeflinemarkup\nwenddeflinemarkup
\LA{}license\RA{}

\LA{}domain DIRRING DirichletRing\RA{}
\nwendcode{}\nwbegindocs{6}\nwdocspar
\end{document}
\nwenddocs{}
