)abbrev package STTAYLOR StreamTaylorSeriesOperations
++ Author: William Burge, Stephen Watt, Clifton J. Williamson
++ Date Created: 1986
++ Basic Operations:
++ Related Domains: Stream(A), ParadoxicalCombinatorsForStreams(A),
++   StreamTranscendentalFunctions(A),
++   StreamTranscendentalFunctionsNonCommutative(A)
++ Also See:
++ AMS Classifications:
++ Keywords: stream, Taylor series
++ Examples:
++ References:
++ Description:
++   StreamTaylorSeriesOperations implements Taylor series arithmetic,
++   where a Taylor series is represented by a stream of its coefficients.
StreamTaylorSeriesOperations(A) : Exports == Implementation where
  A :        Ring
  RN     ==> Fraction Integer
  I      ==> Integer
  NNI    ==> NonNegativeInteger
  ST     ==> Stream
  SP2    ==> StreamFunctions2
  SP3    ==> StreamFunctions3
  L      ==> List
  LA     ==> List A
  YS     ==> Y$ParadoxicalCombinatorsForStreams(A)
  UN     ==> Union(ST A,"failed")
  Exports ==> with
    "+"          : (ST A,ST A) -> ST A
      ++ a + b returns the power series sum of \spad{a} and \spad{b}:
      ++ \spad{[a0, a1, ..] + [b0, b1, ..] = [a0 + b0, a1 + b1, ..]}
    "-"          : (ST A,ST A) -> ST A
      ++ a - b returns the power series difference of \spad{a} and
      ++ \spad{b}: \spad{[a0, a1, ..] - [b0, b1, ..] = [a0 - b0, a1 - b1, ..]}
    "-"          : ST A -> ST A
      ++ - a returns the power series negative of \spad{a}:
      ++ \spad{- [a0, a1, ...] = [- a0, - a1, ...]}
    "*"          : (ST A,ST A) -> ST A
      ++ a * b returns the power series (Cauchy) product of \spad{a} and b:
      ++ \spad{[a0, a1, ...] * [b0, b1, ...] = [c0, c1, ...]} where
      ++ \spad{ck = sum(i + j = k, ai * bk)}.
    "*"          : (A,ST A) -> ST A
      ++ r * a returns the power series scalar multiplication of r by \spad{a}:
      ++ \spad{r * [a0, a1, ...] = [r * a0, r * a1, ...]}
    "*"          : (ST A,A) -> ST A
      ++ a * r returns the power series scalar multiplication of \spad{a} by r:
      ++ \spad{[a0, a1, ...] * r = [a0 * r, a1 * r, ...]}
    "exquo"      : (ST A,ST A) -> Union(ST A,"failed")
      ++ exquo(a, b) returns the power series quotient of \spad{a} by b,
      ++ if the quotient exists, and "failed" otherwise
    "/"          : (ST A,ST A) -> ST A
      ++ a / b returns the power series quotient of \spad{a} by b.
      ++ An error message is returned if \spad{b} is not invertible.
      ++ This function is used in fixed point computations.
    recip        : ST A -> UN
      ++ recip(a) returns the power series reciprocal of \spad{a}, or
      ++ "failed" if not possible.
    monom        : (A, I) -> ST A
      ++ monom(deg, coef) is a monomial of degree deg with coefficient
      ++ coef.
    integers     : I -> ST I
      ++ integers(n) returns \spad{[n, n+1, n+2, ...]}.
    oddintegers  : I -> ST I
      ++ oddintegers(n) returns \spad{[n, n+2, n+4, ...]}.
    int          : A -> ST A
      ++ int(r) returns [r, r+1, r+2, ...], where r is a ring element.
    mapmult      : (ST A, ST A) -> ST A
      ++ mapmult([a0, a1, ..], [b0, b1, ..])
      ++ returns \spad{[a0*b0, a1*b1, ..]}.
    deriv        : ST A -> ST A
      ++ deriv(a) returns the derivative of the power series with
      ++ respect to the power series variable. Thus
      ++ \spad{deriv([a0, a1, a2, ...])} returns \spad{[a1, 2 a2, 3 a3, ...]}.
    gderiv       : (I -> A, ST A)  -> ST A
      ++ gderiv(f, [a0, a1, a2, ..]) returns
      ++ \spad{[f(0)*a0, f(1)*a1, f(2)*a2, ..]}.
    coerce       : A -> ST A
      ++ coerce(r) converts a ring element r to a stream with one element.
    eval         : (ST A, A) -> ST A
      ++ eval(a, r) returns a stream of partial sums of the power series
      ++ \spad{a} evaluated at the power series variable equal to r.
    compose      : (ST A, ST A) -> ST A
      ++ compose(a, b) composes the power series \spad{a} with
      ++ the power series b.
    lagrange     : ST A -> ST A
      ++ lagrange(g) produces the power series for f where f is
      ++ implicitly defined as \spad{f(z) = z*g(f(z))}.
    revert       : ST A -> ST A
      ++ revert(a) computes the inverse of a power series \spad{a}
      ++ with respect to composition.
      ++ the series should have constant coefficient 0 and invertible
      ++ first order coefficient.
    addiag       : ST ST A -> ST A
      ++ addiag(x) performs diagonal addition of a stream of streams. if x =
      ++ \spad{[[a<0, 0>, a<0, 1>, ..], [a<1, 0>, a<1, 1>, ..], [a<2, 0>, a<2, 1>, ..], ..]}
      ++ and \spad{addiag(x) = [b<0, b<1>, ...], then b<k> = sum(i+j=k, a<i, j>)}.
    prodiag : ST ST A -> ST A
      ++ prodiag(x) performs "diagonal" infinite product of a stream
      ++ of streams.  When \spad{x(i)} is interpreted as stream of
      ++ coefficients of series \spad{f_i(z), i=1,...}, then
      ++ \spad{prodiag(x) = (1 + z*f_1(z))*(1 + z^2*f_2(x))*...}
    lambert      : ST A -> ST A
      ++ lambert(st) computes \spad{f(x) + f(x^2) + f(x^3) + ...}
      ++ if st is a stream representing \spad{f(x)}.
      ++ This function is used for computing infinite products.
      ++ If \spad{f(x)} is a power series with constant coefficient 1 then
      ++ \spad{prod(f(x^n), n = 1..infinity) = exp(lambert(log(f(x))))}.
    oddlambert   : ST A -> ST A
      ++ oddlambert(st) computes \spad{f(x) + f(x^3) + f(x^5) + ...}
      ++ if st is a stream representing \spad{f(x)}.
      ++ This function is used for computing infinite products.
      ++ If f(x) is a power series with constant coefficient 1 then
      ++ \spad{prod(f(x^(2*n-1)), n=1..infinity) = exp(oddlambert(log(f(x))))}.
    evenlambert  : ST A -> ST A
      ++ evenlambert(st) computes \spad{f(x^2) + f(x^4) + f(x^6) + ...}
      ++ if st is a stream representing \spad{f(x)}.
      ++ This function is used for computing infinite products.
      ++ If \spad{f(x)} is a power series with constant coefficient 1, then
      ++ \spad{prod(f(x^(2*n)), n=1..infinity) = exp(evenlambert(log(f(x))))}.
    generalLambert : (ST A, I, I) -> ST A
      ++ generalLambert(f(x), a, d) returns
      ++ \spad{f(x^a) + f(x^(a + d)) + f(x^(a + 2 d)) + ...}.
      ++ \spad{f(x)} should have zero constant
      ++ coefficient and \spad{a} and d should be positive.
    general_Lambert_product : (ST A, I, I) -> ST A
      ++ general_Lambert_product(f(x), a, d) returns
      ++ \spad{f(x^a)*f(x^(a + d))*f(x^(a + 2 d))* ...}.
      ++ \spad{f(x)} should have constant coefficient equal to one
      ++ and \spad{a} and d should be positive.
    multisect    : (I, I, ST A) -> ST A
      ++ multisect(a, b, st)
      ++ selects the coefficients of \spad{x^((a+b)*n+a)},
      ++ and changes them to \spad{x^n}.
    invmultisect : (I, I, ST A) -> ST A
      ++ invmultisect(a, b, st) substitutes \spad{x^((a+b)*n)} for \spad{x^n}
      ++ and multiplies by \spad{x^b}.
    if A has Algebra RN then
      integrate  : (A, ST A) -> ST A
        ++ integrate(r, a) returns the integral of the power series \spad{a}
        ++ with respect to the power series variable where
        ++ r denotes the constant of integration. Thus
        ++ \spad{integrate(a, [a0, a1, a2, ...]) = [a, a0, a1/2, a2/3, ...]}.
      lazyIntegrate  : (A, () -> ST A) -> ST A
        ++ lazyIntegrate(r, f) is a version of integrate
        ++ used for fixed point computations.
      nlde       : ST ST A -> ST A
        ++ nlde(u) solves a
        ++ first order non-linear differential equation described by u of the
        ++ form \spad{[[b<0, 0>, b<0, 1>, ...], [b<1, 0>, b<1, 1>, .], ...]}.
        ++ the differential equation has the form
        ++ \spad{y' = sum(i=0 to infinity, j=0 to infinity, b<i, j>*(x^i)*(y^j))}.
      powern : (RN, ST A) -> ST A
        ++ powern(r, f) raises power series f to the power r.
    if A has Field then
      mapdiv     : (ST A, ST A) -> ST A
        ++ mapdiv([a0, a1, ..], [b0, b1, ..]) returns
        ++ \spad{[a0/b0, a1/b1, ..]}.
      lazyGintegrate : (I -> A, A, () -> ST A) -> ST A
        ++ lazyGintegrate(f, r, g) is used for fixed point computations.
      power      : (A, ST A) -> ST A
        ++ power(a, f) returns the power series f raised to the power \spad{a}.

  Implementation ==> add

--% definitions

    zro : () -> ST A
    -- returns a zero power series
    zro() == empty()$ST(A)

--% arithmetic

    x + y == delay
      empty? y => x
      empty? x => y
      eq?(x, rst x) => map(z +-> frst x + z, y)
      eq?(y, rst y) => map(z +-> frst y + z, x)
      concat(frst x + frst y, rst x + rst y)

    x - y == delay
      empty? y => x
      empty? x => -y
      eq?(x, rst x) => map(z +-> frst x - z, y)
      eq?(y, rst y) => map(z +-> z - frst y, x)
      concat(frst x - frst y, rst x - rst y)

    -y == map(z +-> -z, y)

    (s : A) * (x : ST A) ==
      zero? s => zro()
      map(z +-> s*z, x)

    (x : ST A) * (s : A) ==
      zero? s => zro()
      map(z +-> z*s, x)

    iDiv : (ST A, ST A, A) -> ST A
    iDiv(x, rst_y, ry0) == delay
      empty? x => empty()
      c0 := frst x * ry0
      concat(c0, iDiv(rst x - c0*rst_y, rst_y, ry0))

    i_div1 : (ST A, ST A, A, A) -> ST A
    i_div1(rst_x, rst_y, ry0, c0) == delay
        empty?(rst_y) => map(z +-> z*ry0, rst_x)
        iDiv(rst_x - c0*rst_y, rst_y, ry0)

    x exquo y ==
      y0 : A
      for n in 1.. repeat
        n > 1000 => return "failed"
        empty? y => return "failed"
        empty? x => return empty()
        (y0 := frst y) = 0 =>
          frst x = 0 => (x := rst x; y := rst y)
          return "failed"
        -- first entry in y is non-zero
        break
      (ry0u := recip y0) case "failed" => "failed"
      ry0 := ry0u@A
      x0 := frst x
      c0 := x0*ry0
      concat(c0, i_div1(rst x, rst y, ry0, c0))

    (x : ST A) / (y : ST A) == delay
      empty? y => error "/: division by zero"
      empty? x => empty()
      (ry0u := recip frst y) case "failed" =>
        error "/: second argument is not invertible"
      ry0 := ry0u@A
      x0 := frst x
      c0 := x0*ry0
      concat(c0, i_div1(rst x, rst y, ry0, c0))

    recip x ==
      empty? x => "failed"
      rh1 := recip frst x
      rh1 case "failed" => "failed"
      rh := rh1@A
      delay
        concat(rh, iDiv(- rh * rst x, rst x, rh))

--% coefficients

    rp : (I, A) -> L A
    -- rp(z, s) is a list of length z each of whose entries is s.
    rp(z, s) ==
      z <= 0 => empty()
      concat(s, rp(z-1, s))

    rpSt : (I, A) -> ST A
    -- rpSt(z, s) is a stream of length z each of whose entries is s.
    rpSt(z, s) == delay
      z <= 0 => empty()
      concat(s, rpSt(z-1, s))

    monom(s, z) ==
      z < 0 => error "monom: cannot create monomial of negative degree"
      concat(rpSt(z, 0), concat(s, zro()))

--% some streams of integers
    nnintegers : NNI -> ST NNI
    nnintegers zz == stream(y +-> y + 1, zz)
    integers z == stream(y +-> y + 1, z)
    oddintegers z == stream(y +-> y + 2, z)
    int s == stream(y +-> y + 1, s)

--% multiplication

    stmult(n0 : Integer, x0 : ST A, y0 : ST A, _
           ll0 : List A) : ST A == delay
        x := x0
        y := y0
        n := n0
        ll := ll0
        if empty? y then
             n < 0 => return empty()
             empty? x => return empty()
             x := rst x
        else
             c := frst y
             y := rst y
             if n < 0 and c = 0 then
                 return concat(0, stmult(n, x, y, ll))
             else
                 ll := cons(c, ll)
                 n := n + 1
        res : A := 0
        xp := x
        llp := ll
        for i in 0..n repeat
            empty? xp =>
                i = 0 => return empty()
                break
            res := res + frst xp * first(llp)
            llp := rest(llp)
            xp := rst xp
        -- make sure that when we multiply finite streams
        -- tail is explicitly empty
        explicitlyEmpty? rst x and
          explicitlyEmpty? y => concat(res, empty())
        concat(res, stmult(n, x, y, ll))

    (x : ST A) * (y : ST A) == delay
        empty? y => zro()
        empty? x => zro()
        stmult(-1, x, y, [])

--% derivatives

    mapmult(x, y) == delay
      empty? y => zro()
      empty? x => zro()
      concat(frst x * frst y, mapmult(rst x, rst y))

    deriv x ==
      empty? x => zro()
      mapmult(int 1, rest x)

    gderiv(f, x) ==
      empty? x => zro()
      mapmult(map(f, integers 0)$SP2(I, A), x)

--% coercions

    coerce(s : A) ==
      zero? s => zro()
      concat(s, zro())

--% evaluations and compositions

    eval(x, at) == scan(0, (y, z) +-> y + z,
                       mapmult(x, stream(y +-> at * y, 1)))$SP2(A, A)

    compose(x, y) == delay
      empty? y => concat(frst x, zro())
      not zero? frst y =>
        error "compose: 2nd argument should have 0 constant coefficient"
      empty? x => zro()
      concat(frst x, compose(rst x, y) * rst(y))

--% reversion

    lagrangere : (ST A, ST A) -> ST A
    lagrangere(x, c) == delay(concat(0, compose(x, c)))
    lagrange x == YS(y +-> lagrangere(x, y))

    revert x ==
      empty? x => error "revert: should be nonzero"
      zero? frst x =>
        empty? rst x => error "revert: should be nonzero"
        y := recip(rst x)
        y case ST(A) =>
            lagrange(y@ST(A))
        error "revert: should start 0, x, ... with invertible x"
      error "revert: should start 0, x, ..."

    -- prodiag1([f_n, ...], n) computes
    -- ((1 + z^n*f_n)*(1 + z^{n+1}*f_{n+1})... - 1)/z^n
    prodiag1(ststa : ST ST A, n : Integer) : ST(A) == delay
        empty? ststa => zro()
        f1 := frst ststa
        r1 := rst ststa
        p1 := prodiag1(r1, n + 1)
        empty? f1 => concat(0, p1)
        concat(frst(f1), rst(f1) + p1 + concat(rpSt(n, 0), f1*p1))

    prodiag(ststa : ST ST A) == concat(1, prodiag1(ststa, 1))

--% lambert functions

    addiag(ststa : ST ST A) == delay
      empty? ststa => zro()
      empty? frst ststa => concat(0, addiag rst ststa)
      concat(frst(frst ststa), rst(frst ststa) + addiag(rst ststa))

-- lambert operates on a series +/[a[i]x^i for i in 1..] , and produces
-- the series +/[a[i](x^i/(1-x^i)) for i in 1..] i.e. forms the
-- coefficients A[n] which is the sum of a[i] for all divisors i of n
-- (including 1 and n)

    rptg1 : (I, A) -> ST A
    --                               ---------
    -- returns the repeating stream [s, 0, ..., 0]; (there are z zeroes)
    rptg1(z, s) == repeating concat(s, rp(z, 0))

    rptg2 : (I, A) -> ST A
    --                                       ---------
    -- returns the repeating stream [0, ..., 0, s, 0, ..., 0]
    -- there are z leading zeroes and z-1 in the period
    rptg2(z, s) == repeating concat(rp(z, 0), concat(s, rp(z-1, 0)))

    rptg3 : (I, I, I, A) -> ST A
    rptg3(a, d, n, s) ==
      concat(rpSt(n*(a-1), 0), repeating(concat(s, rp(d*n-1, 0))))

    lambert x == delay
      empty? x => zro()
      zero? frst x =>
        concat(0, addiag(map(rptg1, integers 0, rst x)$SP3(I, A, ST A)))
      error "lambert:constant coefficient should be zero"

    oddlambert x == delay
      empty? x => zro()
      zero? frst x =>
        concat(0, addiag(map(rptg1, oddintegers 1, rst x)$SP3(I, A, ST A)))
      error "oddlambert: constant coefficient should be zero"

    evenlambert x == delay
      empty? x => zro()
      zero? frst x =>
        concat(0, addiag(map(rptg2, integers 1, rst x)$SP3(I, A, ST A)))
      error "evenlambert: constant coefficient should be zero"

    generalLambert(st, a, d) == delay
      a < 1 or d < 1 =>
        error "generalLambert: both integer arguments must be positive"
      empty? st => zro()
      zero? frst st =>
        concat(0, addiag(map((x, y) +-> rptg3(a, d, x, y), _
                 integers 1, rst st)$SP3(I, A, ST A)))
      error "generalLambert: constant coefficient should be zero"

    rptg4(a : I, d : I, st : ST A) : ST A == delay
        empty? st => zro()
        concat(rpSt(a, 0),  concat(frst(st), rptg4(d, d, rst(st))))

    general_Lambert_product(st, a, d) == delay
        a < 1 or d < 1 => error
          "general_Lambert_product: both integer arguments must be positive"
        empty? st or (frst st ~= 1) => error
          "general_Lambert_product: constant coefficient should be one"
        st1 := rst st
        prodiag(map(n +-> rptg4(a + n*d - n - 1, a + n*d - 1, st1),
                 integers 0)$SP2(I, ST A))

--% misc. functions

    ms : (I, I, ST A) -> ST A
    ms(m, n, s) == delay
      empty? s => zro()
      zero? n => concat(frst s, ms(m, m-1, rst s))
      ms(m, n-1, rst s)

    multisect(b, a, x) == ms(a+b, 0, rest(x, a :: NNI))

    altn : (ST A, ST A) -> ST A
    altn(zs, s) == delay
      empty? s => zro()
      concat(frst s, concat(zs, altn(zs, rst s)))

    invmultisect(a, b, x) ==
      concat(rpSt(b, 0), altn(rpSt(a + b - 1, 0), x))

-- comps(ststa, y) forms the composition of +/b[i, j]*y^i*x^j
-- where y is a power series in y.

    cssa ==> concat$(ST ST A)
    mapsa ==> map$SP2(ST A, ST A)
    comps : (ST ST A, ST A) -> ST ST A
    comps(ststa, x) == delay$(ST ST A)
       empty? ststa => empty()$(ST ST A)
       empty? x => cssa(frst ststa, empty()$(ST ST A))
       cssa(frst ststa, mapsa(y +-> (rst x)*y, comps(rst ststa, x)))

    if A has Algebra RN then
      integre : (ST A, I) -> ST A
      integre(x, n) == delay
        empty? x => zro()
        concat((1$I/n) * frst(x), integre(rst x, n + 1))

      integ : ST A -> ST A
      integ x == integre(x, 1)

      integrate(a, x) == concat(a, integ x)
      lazyIntegrate(s, xf) == concat(s, integ(delay xf))

      nldere : (ST ST A, ST A) -> ST A
      nldere(lslsa, c) == lazyIntegrate(0, addiag(comps(lslsa, c)))
      nlde lslsa == YS(y +-> nldere(lslsa, y))

      RATPOWERS : Boolean := A has "^": (A,RN) -> A

      smult : (RN, ST A) -> ST A
      smult(rn, x) == map(y +-> rn*y, x)
      powerrn : (RN, ST A, ST A) -> ST A
      powerrn(rn, x, c) == delay
        concat(1, integ(smult(rn + 1, c * deriv x)) - rst x * c)
      powern(rn, x) ==
        order : I := 0
        for n in 0.. repeat
          empty? x => return zro()
          not zero? frst x => (order := n; break)
          x := rst x
          n = 1000 =>
            error "^: series with many leading zero coefficients"
        (ord := (order exquo denom(rn))) case "failed" =>
          error "^: rational power does not exist"
        co := frst x
        if ord > 0 and rn < 0 then
           error "^: negative power does not exist"
        (invCo := recip co) case "failed" =>
           error "^ rational power of coefficient undefined"
-- This error message is misleading, isn't it? see sups.spad/cRationalPower
        power :=
          (co = 1) => YS(y +-> powerrn(rn, x, y))
          (denom rn) = 1 =>
            not negative?(num := numer rn) =>
-- It seems that this cannot happen, but I don't know why
              (co^num::NNI) * YS(y +-> powerrn(rn, (invCo::A)*x, y))
            (invCo::A)^((-num)::NNI) * YS(y +-> powerrn(rn, (invCo::A)*x, y))

          RATPOWERS => co^rn * YS(y +-> powerrn(rn, (invCo::A)*x, y))
          error "^ rational power of coefficient undefined"
        monom(1, (ord :: I) * numer(rn)) * power

    if A has Field then
      mapdiv(x, y) == delay
        empty? y => error "stream division by zero"
        empty? x => zro()
        concat(frst x/frst y, mapdiv(rst x, rst y))

      ginteg : (I -> A, ST A) -> ST A
      ginteg(f, x) == mapdiv(x, map(f, integers 1)$SP2(I, A))

      lazyGintegrate(fntoa, s, xf) == concat(s, ginteg(fntoa, delay xf))

      finteg : ST A -> ST A
      finteg x == mapdiv(x, int 1)
      powerre : (A, ST A, ST A) -> ST A
      powerre(s, x, c) == delay
        empty? x => zro()
        frst x ~= 1 => error "^:constant coefficient should be 1"
        concat(frst x, finteg((s+1)*(c*deriv x))-rst x * c)
      power(s, x) == YS(y +-> powerre(s, x, y))

--Copyright (c) 1991-2002, The Numerical ALgorithms Group Ltd.
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--      notice, this list of conditions and the following disclaimer in
--      the documentation and/or other materials provided with the
--      distribution.
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--    - Neither the name of The Numerical ALgorithms Group Ltd. nor the
--      names of its contributors may be used to endorse or promote products
--      derived from this software without specific prior written permission.
--
--THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
--IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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