source: git/Singular/LIB/rootsur.lib @ 1f9a84

///////////////////////////////////////////////////////////////////////////////
version="$Id$";
category="Teaching";
info="
LIBRARY: rootsur.lib   Counting number of real roots of univariate polynomial
AUTHOR:                 Enrique A. Tobis, etobis@dc.uba.ar

OVERVIEW:  Routines for bounding and counting the number of real roots of a
           univariate polynomial, by means of several different methods, namely
           Descartes' rule of signs, the Budan-Fourier theorem, Sturm sequences
           and Sturm-Habicht sequences. The first two give bounds on the number
           of roots. The other two compute the actual number of roots of the
           polynomial. There are several wrapper functions, to simplify the
           application of the aforesaid theorems and some functions
           to determine whether a given polynomial is univariate.
           References: Basu, Pollack, Roy, \"Algorithms in Real Algebraic
           Geometry\", Springer, 2003.


PROCEDURES:
  isuni(p)         Checks whether a polynomial is univariate
  whichvariable(p) The only variable of a univariate monomial (or 0)
  varsigns(p)      Number of sign changes in a list
  boundBuFou(p,a,b) Bound for number of real roots of polynomial p in interval (a,b)
  boundposDes(p)   Bound for the number of positive real roots of polynomial p
  boundDes(p)      Bound for the number of real roots of polynomial p
  allrealst(p)     Checks whether all the roots of a polynomial are real (via Sturm)
  maxabs(p)        A bound for the maximum absolute value of a root of a poly
  allreal(p)       Checks whether all the roots of a polynomial are real (via St-Ha)
  sturm(p,a,b)     Number of real roots of a polynomial on an interval (via Sturm)
  sturmseq(p)      Sturm sequence of a polynomial
  sturmha(p,a,b)   Number of real roots of a polynomial in (a,b) (via Sturm-Habicht)
  sturmhaseq(p)    A Sturm-Habicht Sequence of a polynomial
  reverse(l)       Reverses a list
  nrroots(p)       The number of real roots of p
  isparam(p)       Returns 0 if and only if the polynomial has non-parametric coefficients

KEYWORDS:         real roots, univariate polynomial
";
///////////////////////////////////////////////////////////////////////////////

static proc isparametric(poly p)
{
  int ispar;
  def ba = basering;

  // If the basering has parameters declared
  if (npars(basering) != 0) {
    // If we were given just a polynomial
    list lba = ringlist(ba);
    lba[1]=0;
    def rba = ring(lba); setring rba;
    poly p1 = imap(ba,p);
    setring ba;
    poly p1 = imap(rba,p1);
    ispar = (size(p-p1)!=0);
  }
  return (ispar);
}
///////////////////////////////////////////////////////////////////////////////
proc isparam(list #)
"USAGE:     isparam(ideal/module/poly/list);
RETURN:    int: 0 if the argument has non-parametric coefficients and 1 if it
           has parametric coefficients
EXAMPLE:   example isparam; shows an example"
{
  int i;
  int ispar;
  def ar = #[1];

  // It we were given only one argument (not a list)
  if (size(#) == 1) {
    if (typeof(ar) == "number") {
      ispar = (pardeg(ar) > 0);
    } else {
    if (typeof(ar) == "poly") {
      ispar = isparametric(ar);
    } else {
    if (typeof(ar) == "ideal" || typeof(ar) == "module") {
      // Ciclo que revisa cada polinomio
      i = size(ar);
      while (!ispar && (i >= 1)) {
        ispar = ispar || (isparametric(ar[i]));
        i--;
      }
    } else {
    if (typeof(ar) == "matrix" || typeof(ar) == "intmat") {
      int j;
      i = nrows(ar);
      while (!ispar && (i >= 1)) {
        j = nrows(ar);
        while (!ispar && (j >= 1)) {
          ispar = ispar || (isparametric(ar[i,j]));
          j--;
        }
        i--;
      }
    }
  }}}} else {
  if (size(#) > 1) {
    i = size(#);
    while (!ispar && (i >= 1)) {
      if ((typeof(#[i]) != "poly") && (typeof(#[i]) != "number") &&
          typeof(#[i]) != "int") {
              ERROR("This procedure only works with lists of polynomials");
      }
      ispar = ispar || (isparametric(#[i]));
      i--;
    }
  }}
  return (ispar);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  isparam(2x3-56x+2);
  ring s = (0,a,b,c),x,dp;
  isparam(2x3-56x+2);
  isparam(2x3-56x+abc);
}
///////////////////////////////////////////////////////////////////////////////
proc isuni(poly p)
"USAGE:     isuni(p); poly p;
RETURN:    poly: if p is a univariate polynomial, it returns the variable. If
           not, zero.
SEE ALSO:  whichvariable
EXAMPLE:   example isuni; shows an example"
{
  int v=univariate(p);
  if (v== -1) { v=1; }
  if (v>0) { return(var(v)); }
  else     { return(0); }
}
example
{
  echo = 2;
  ring r = 0,(x,y),dp;
  poly p = 6x7-3x2+2x-15/7;
  isuni(p);
  isuni(p*y);
}
///////////////////////////////////////////////////////////////////////////////
proc whichvariable(poly p)
"USAGE:     whichvariable(p); poly p
RETURN:    poly: if p is a univariate monomial, the variable. Otherwise 0.
ASSUME:    p is a monomial
SEE ALSO:  isuni
EXAMPLE:   example whichvariable; shows an example"
{
  if (size(p) != 1)
  { ERROR("p must be a monomial"); }
  int v=univariate(p);
  if (v== -1) { v=1; }
  if (v>0) { return(var(v)); }
  else     { return(0); }
}
example
{
  echo = 2;
  ring r = 0,(x,y),dp;
  whichvariable(x5);
  whichvariable(x3y);
}
///////////////////////////////////////////////////////////////////////////////
proc varsigns(list l)
"USAGE:     varsigns(l); list l.
RETURN:    int: the number of sign changes in the list l
SEE ALSO:  boundposDes
EXAMPLE:   example varsigns; shows an example"
{
  int lastsign;
  int numberofchanges = 0;

  if (isparam(l)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  lastsign = sign(l[1]);

  for (int i = 1; i <= size(l); i = i + 1) {
    if (sign(l[i]) != lastsign && sign(l[i]) != 0) {
      numberofchanges++;
      lastsign = sign(l[i]);
    }
  }
  return (numberofchanges);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  list l = 1,2,3;
  varsigns(l);
  l = 1,-1,2,-2,3,-3;
  varsigns(l);
}
///////////////////////////////////////////////////////////////////////////////
proc boundBuFou(poly p,number a,number b)
"USAGE:     boundBuFou(p,a,b); p poly, a,b number
RETURN:    int: an upper bound for the number of real roots of p in (a,b],
           with the same parity as the actual number of roots (using the
           Budan-Fourier Theorem)
ASSUME:    - p is a univariate polynomial with rational coefficients@*
           - a, b are rational numbers with a < b
SEE ALSO:  boundposDes,varsigns
EXAMPLE:   example boundBuFou; shows an example"
{
  int i;
  poly variable;
  list Der;
  list Dera,Derb;
  int d;
  number bound;

  variable = isuni(p);

  if (isparam(p) || isparam(a) || isparam(b)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  // p must be a univariate polynomial
  if (variable == 0) {
    ERROR("p must be a univariate polynomial");
  }

  if (a >= b) {
    ERROR("a must be smaller than b");
  }

  d = deg(p);

  // We calculate the list of derivatives

  Der[d+1] = p;

  for (i = 0;i < d;i++) {
    Der[d-i] = diff(Der[d-i+1],variable);
  }

  // Then evaluate that list

  for (i = d+1;i >= 1;i--) {
    Dera [i] = leadcoef(subst(Der[i],variable,a));
    Derb [i] = leadcoef(subst(Der[i],variable,b));
  }

  // Finally we calculate the sign variations

  bound = varsigns(Dera) - varsigns(Derb);

  return(bound);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  boundBuFou(p,-3,5);
  boundBuFou(p,-2,5);
}
///////////////////////////////////////////////////////////////////////////////
proc boundposDes(poly p)
"USAGE:     boundposDes(p); poly p
RETURN:    int: an upper bound for the number of positive roots of p, with
           the same parity as the actual number of positive roots of p.
ASSUME:    p is a univariate polynomial with rational coefficients
SEE ALSO:  boundBuFou
EXAMPLE:   example boundposDes; shows an example"
{
  poly g;
  number nroots;
  poly variable;
  list coefficients;
  int i;

  variable = isuni(p);

  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  // p must be a univariate polynomial
  if (variable == 0) {
    ERROR("p must be a univariate polynomial");
  }

  g = p; // We will work with g

  // We check whether 0 is a root of g, and if so, remove it
  if (subst(g,variable,0) == 0) {
    g = g/variable^(deg(g[size[g]]));
  }

  // We count the number of positive roots
  i = size(g);
  while (i >= 1) {
    coefficients[i] = leadcoef(g[i]);
    i--;
  }
  nroots = varsigns(coefficients);

  return(nroots);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  boundposDes(p);

  p = p*(x2+1);

  boundposDes(p);
}
///////////////////////////////////////////////////////////////////////////////
proc boundDes(poly p)
"USAGE:     boundDes(p); poly p
RETURN:    int: an upper bound for the number of real roots of p, with
           the same parity as the actual number of real roots of p.
ASSUME:    p is a univariate polynomial with rational coefficients
SEE ALSO:  boundBuFou
EXAMPLE:   example boundDes; shows an example"
{
  poly g;
  number nroots;
  poly variable;
  list coefficients;
  int i;

  variable = isuni(p);

  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  // p must be a univariate polynomial
  if (variable == 0) {
    ERROR("p must be a univariate polynomial");
  }

  g = p; // We will work with g

  nroots = 0;
  // We check whether 0 is a root of g, and if so, remove it
  if (subst(g,variable,0) == 0) {
    g = g/variable^(deg(g[size[g]]));
    nroots++;
  }

  // We count the number of positive roots
  i = size(g);
  while (i >= 1) {
    coefficients[i] = leadcoef(g[i]);
    i--;
  }
  nroots = nroots + varsigns(coefficients);

  // We count the number of negative roots
  g = subst(g,variable,-variable);
  i = size(g);
  while (i >= 1) {
    coefficients[i] = leadcoef(g[i]);
    i--;
  }
  nroots = nroots + varsigns(coefficients);

  return(nroots);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  boundDes(p);

  p = p*(x2+1);

  boundDes(p);
}
///////////////////////////////////////////////////////////////////////////////
proc allrealst(poly p)
"USAGE:     allrealst(p); poly p
RETURN:    int: 1 if and only if all the roots of p are real, 0 otherwise.
           Checks by using Sturm's Theorem whether all the roots of p are real
ASSUME:    p is a univariate polynomial with rational coefficients
SEE ALSO:  allreal,sturm,sturmha
EXAMPLE:   example allrealst; shows an example"
{
  number upper,lower;
  poly sqfp; // The square-free part of p
  poly variable;

  variable = isuni(p);

  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }
  if (variable == 0) {
    ERROR ("p must be a univariate polynomial");
  }

  sqfp = p/gcd(p,diff(p,variable));

  upper = maxabs(sqfp); // By adding one we ensure that sqfp(upper) != 0
  lower = -upper;

  return (sturm(sqfp,lower,upper) == deg(sqfp));
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  allrealst(p);
  p = p*(x2+1);
  allrealst(p);
}
///////////////////////////////////////////////////////////////////////////////
proc maxabs(poly p)
"USAGE:     maxabs(p); poly p
RETURN:    number: an upper bound for the largest absolute value of a root of p
ASSUME:    p is a univariate polynomial with rational coefficients
SEE ALSO:  sturm
EXAMPLE:   example maxabs; shows an example"
{
  number maximum;
  poly monic;
  int i;

  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  monic =  simplify(p,1);

  maximum = 0;

  for (i = 1; i <= size(monic); i = i + 1) {
    maximum = max(abs(leadcoef(p[i])),maximum);
  }

  return (maximum + 1);
}
example
{
  echo = 2;
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  maxabs(p);
}
///////////////////////////////////////////////////////////////////////////////
proc sturm(poly p,number a,number b)
"USAGE:     sturm(p,a,b); poly p, number a,b
RETURN:    int: the number of real roots of p in (a,b]
ASSUME:    p is a univariate polynomial with rational coefficients,@*
           a, b are rational numbers with a < b
SEE ALSO:  sturmha,allrealst,allreal
EXAMPLE:   example sturm; shows an example"
{
  list l;
  list pa;
  list pb;
  int signsA,signsB;
  int i;
  int nroots;
  poly variable;

  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  variable = isuni(p);

  if (variable == 0) {
    ERROR ("p must be a univariate polynomial");
  }

  if (a >= b) {
    ERROR("a must be lower than b");
  }

  if (subst(p,variable,a) == 0 || subst(p,variable,b) == 0) {
    ERROR ("Neither a nor b can be roots of P");
  }

  l = sturmseq(p);

  i = size(l);

  while (i >= 1) { // We build the sequences
    pa[i] = leadcoef(subst(l[i],variable,a));
    pb[i] = leadcoef(subst(l[i],variable,b));
    i--;
  }

  signsA = varsigns(pa);
  signsB = varsigns(pb);

  nroots = signsA - signsB + nroots;

  return (nroots);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  sturm(p,-3,6);
  p = p*(x2+1);
  sturm(p,-3,6);
  p = p*(x+2);
  sturm(p,-3,6);
}
///////////////////////////////////////////////////////////////////////////////
proc sturmseq(poly p)
"USAGE:     sturmseq(p); p poly
RETURN:    list: a Sturm sequence of p
ASSUME:    p is a univariate polynomial with rational coefficients
THEORY:    The Sturm sequence of p (also called remainder sequence) is the
           sequence beginning with p, p' and goes on with the negative part of
           the remainder of the two previous polynomials, until the remainder
           is zero.
           See: Basu, Pollack, Roy, Algorithms in Real Algebraic Geometry,
           Springer, 2003.
SEE ALSO:  sturm,sturmhaseq
EXAMPLE:   example sturmseq; shows an example"
{
  list stseq;
  poly variable;
  int i;

  variable = isuni(p);

  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  if (variable == 0) {
    ERROR ("p must be a univariate polynomial");
  }

  // The two first polynomials in Sturm's sequence
  stseq = list();
  stseq[1] = p;
  stseq[2] = diff(p,variable);

  poly q = -reduce(stseq[1],std(stseq[2]));
  i = 3;

  while (q <> 0) {
    stseq[i] = q;
    q = -reduce(stseq[i-1],std(stseq[i]));
    i++;
  }

  // Right now, we have gcd(P,P') in stseq[size(stseq)];

  for (i = size(stseq)-1;i >= 1;i--) {
    stseq[i] = stseq[i]/(sign(leadcoef(stseq[size(stseq)]))*stseq[size(stseq)]);
    stseq[i] = stseq[i]/abs(leadcoef(stseq[i]));
  }

  // We divide the gcd by itself
  stseq[size(stseq)] = sign(leadcoef(stseq[size(stseq)]));

  return (stseq);
}
example
{
  echo = 2;
  ring r = 0,(z,x),dp;
  poly p = x5-3x4+12x3+7x-153;
  sturmseq(p);
}
///////////////////////////////////////////////////////////////////////////////
proc allreal(poly p)
"USAGE:     allreal(p);
RETURN:    int: 1 if and only if all the roots of p are real, 0 otherwise
SEE ALSO:  allrealst
EXAMPLE:   example allreal; shows an example"
{
  number upper,lower;
  poly sqfp; // The square-free part of p
  poly variable;

  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  variable = isuni(p);

  if (variable == 0) {
    ERROR ("p must be a univariate polynomial");
  }

  sqfp = p/gcd(p,diff(p,variable));

  return (sturmha(sqfp,-maxabs(p),maxabs(p)) == deg(sqfp));
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  allreal(p);
  p = p*(x2+1);
  allreal(p);
}
///////////////////////////////////////////////////////////////////////////////
proc sturmha(poly P,number a,number b)
"USAGE:     sturmha(p,a,b); poly p, number a,b
RETURN:    int: the number of real roots of p in (a,b) (using a Sturm-Habicht sequence)
SEE ALSO:  sturm,allreal
EXAMPLE:   example sturmha; shows an example"
{
  list seq;
  int i;
  list seqa,seqb;
  poly variable;
  number bound;
  //number result;
  int result;

  if (isparam(P) || isparam(a) || isparam(b)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  variable = isuni(P);

  if (variable == 0) {
    ERROR ("P must be a univariate polynomial");
  }

  if (a >= b) {
    ERROR("a must be lower than b");
  }

  if (subst(P,variable,a) == 0 || subst(P,variable,b) == 0) {
    ERROR ("Neither a nor b can be roots of P");
  }

  seq = sturmhaseq(P);

  bound = maxabs(P);

  if (a < -bound) {
    a = -bound;
  }

  if (b > bound) {
    b = bound;
  }

//  if (a == -bound && b == bound) {
//    for (i = size(seq);i >= 1;i--) {
//      seq[i] = leadcoef(seq[i]);
//    }
//    result = D(seq);
//  } else {
    for (i = size(seq);i >= 1;i--) {
      seqa[i] = leadcoef(subst(seq[i],variable,a));
      seqb[i] = leadcoef(subst(seq[i],variable,b));
    }
    result = (W(seqa) - W(seqb));
//  }
  return (result);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  sturmha(p,-3,6);
  p = p*(x2+1);
  sturmha(p,-3,6);
}
///////////////////////////////////////////////////////////////////////////////
proc sturmhaseq(poly P)
"USAGE:     sturmhaseq(P); P poly.
RETURN:    list: the non-zero polynomials of the Sturm-Habicht sequence of P
ASSUME:    P is a univariate polynomial.
THEORY:    The Sturm-Habicht sequence (also subresultant sequence) is closely
           related to the Sturm sequence, but behaves better with respect to
           the size of the coefficients. It is defined via subresultants.
           See: Basu, Pollack, Roy, Algorithms in Real Algebraic Geometry,
           Springer, 2003.
SEE ALSO:  sturm,sturmseq,sturmha
EXAMPLE:   example sturmhaseq; shows an example"
{
  poly Q;
  poly variable;
  int p,q,i,j,k,l;
  list SR;
  list sr;
  list srbar;
  list T;

  if (isparam(P)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  variable = isuni(P);

  if (variable == 0) {
    ERROR ("P must be a univariate polynomial");
  }

  p = deg(P);
  Q = diff(P,variable);
  q = deg(Q);

  // Initialization
  SR[p+2] = sign(leadcoef(P)^(p-q-1))*P;
//  T[p+2] = SR[p+2];

  srbar[p+2] = sign(leadcoef(P)^(p-q));
  sr[p+2] = srbar[p+2];

  SR[p-1+2] = sign(leadcoef(P)^(p-q+1))*Q;
//  T[p-1+2] = SR[p-1+2];
  srbar[p-1+2] = sign(leadcoef(P)^(p-q+1))*leadcoef(Q);

  i = p+1;
  j = p;

  while (SR[j-1+2] != 0) {
    k = deg(SR[j-1+2]);
    if (k == j-1) {
      sr[j-1+2] = srbar[j-1+2];
      SR[k-1+2] = -(reduce(sr[j-1+2]^2*SR[i-1+2],
                    std(SR[j-1+2])))/(sr[j+2]*srbar[i-1+2]);

//      T[k-1+2] = SR[k-1+2];
      srbar[k-1+2] = leadcoef(SR[k-1+2]);
    }
    if (k < j-1) {
      // Computation of sr[k+2]
      for (l = 1;l <= j-k-1;l++) {
        srbar[j-l-1+2] = ((-1)^l)*(srbar[j-1+2]*srbar[j-l+2])/sr[j+2];
    }
    sr[k+2] = srbar[k+2];

      // Computation of SR[k-1+2]
      SR[k-1+2] = -reduce(srbar[j-1+2]*sr[k+2]*SR[i-1+2],
                  std(SR[j-1+2]))/(sr[j+2]*srbar[i-1+2]);

      srbar[k-1+2] = leadcoef(SR[k-1+2]);

      SR[k+2] = SR[j-1+2] * ( sr[k+2] / leadcoef(SR[j-1+2]));
    }
    i = j;
    j = k;
  }

  // We build a new list, discarding the undefined and zero elements
  // Plus, we reverse the elements

  list filtered;
  i = size(SR);
  while (i >= 1) {
    if (typeof(SR[i]) != "none") {
      if (SR[i] != 0) {
        filtered = insert(filtered,SR[i]);
      }
    }
    i--;
  }

  return (filtered);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = x5-x4+x-3/2;
  list l = sturmhaseq(p);
  l;
}
///////////////////////////////////////////////////////////////////////////////
proc nrroots(poly p)
"USAGE:     nrroots(p); poly p
RETURN:    int: the number of real roots of p
SEE ALSO:  boundposDes, sturm, sturmha
EXAMPLE:   example nrroots; shows an example"
{
  if (isparam(p)) {
    ERROR("This procedure cannot operate with parametric arguments");
  }

  number a = maxabs(p);

  return (sturmha(p,-a,a));

}
example
{
  echo = 2;
  ring r = 0,x,dp;
  poly p = (x+2)*(x-1)*(x-5);
  nrroots(p);
  p = p*(x2+1);
  nrroots(p);
}
///////////////////////////////////////////////////////////////////////////////
static proc abs(number x)
  // Returns the absolute value of x
{
  number av;

  if (x >= 0) {
    av = x;
  } else {
    av = -x;
  }

  return (av);
}
///////////////////////////////////////////////////////////////////////////////
proc sign(number x)
{
  int sgn;

  if (isparam(x)) {
    print(x);
    ERROR("This procedure cannot operate with parameters");
  }

  if (x > 0) {
    sgn = 1;
  } else { if (x < 0) {
    sgn = -1;
  } else {
    sgn = 0;
  }}

  return (sgn);
}
///////////////////////////////////////////////////////////////////////////////
static proc max(number a,number b)
{
  number maximum;

  if (isparam(a) || isparam(b)) {
    ERROR("This procedure cannot operate with parameters");
  }

  if (a > b) {
    maximum = a;
  } else {
    maximum = b;
  }

  return (maximum);
}
///////////////////////////////////////////////////////////////////////////////
proc reverse(list l)
"USAGE:     reverse(l); l list
RETURN:    list: l reversed.
EXAMPLE:   example reverse; shows an example"
{
  int i;
  list result;

  for (i = 1;i <= size(l);i++) {
    result = list(l[i]) + result;
  }
  return (result);
}
example
{
  echo = 2;
  ring r = 0,x,dp;
  list l = 1,2,3,4,5;
  list rev = reverse(l);
  l;
  rev;
}
///////////////////////////////////////////////////////////////////////////////
static proc D(list l)
{
  int p;
  int q;
  int i;
  int sc; // The modified number of sign changes

  if (l[size(l)] == 0) {
    ERROR("l[size(l)] cannot be 0");
  }

  sc = 0;

  // We know that l[size(l)]] != 0
  p = size(l);
  q = p - 1;

  while (searchnot(l,q,-1,0)) {
    q = searchnot(l,q,-1,0);
    if ((p - q) % 2 == 1) { // if p-q is odd
      sc = sc + ((-1)^(((p-q)*(p-q-1)) / 2))*sign(l[p]*l[q]);
    }
    p = q;
    q = p - 1;
  }

  return (sc);
}
///////////////////////////////////////////////////////////////////////////////
static proc search(list l,int from,int dir,number element)
{
  int i;
  int result;
  i = from;

  result = 0;

  while (i + dir >= 0 && i + dir <= size(l) + 1 && !result) {
    if (l[i] == element) {
      result = i;
    }
    i = i + dir;
  }

  return (result);
}
///////////////////////////////////////////////////////////////////////////////
static proc searchnot(list l,int from,int dir,number element)
{
  int i;
  int result;
  i = from;

  result = 0;

  while (i + dir >= 0 && i + dir <= size(l) + 1 && !result) {
    if (l[i] != element) {
      result = i;
    }
    i = i + dir;
  }

  return (result);
}
///////////////////////////////////////////////////////////////////////////////
static proc W(list l)
{
  int i,temp,sc,lastsign,nofzeros,n;

  n = size(l);
  sc = 0;
  nofzeros = 0;
  i = 1;
  lastsign = sign(l[i]);

  i++;

  while (i <= n) {
    if (l[i] == 0) {
      nofzeros++;
    } else {
      temp = lastsign * sign(l[i]);

      if (temp < 0) {
        sc++;
      } else {
        if (nofzeros == 2) {
          sc = sc + 2;
        }
      }
      nofzeros = 0;
      lastsign = temp div lastsign;
    }
    i++;
  }
  return (sc);
}
///////////////////////////////////////////////////////////////////////////////