/// <summary>
/// Expand a power expression.
/// </summary>
/// <param name="f"></param>
/// <param name="x"></param>
/// <returns></returns>
private static Expression ExpandPower(Power f, Expression x)
{
// Get integral exponent of f.
int n = Power.IntegralExponentOf(f);
// If this is an an integral constant negative exponent, attempt to use partial fractions.
if (n < 0 && !ReferenceEquals(x, null))
{
Expression b = f.Left.Factor(x);
if (n != -1)
{
b = Power.New(b, Math.Abs(n));
}
return(ExpandPartialFractions(1, b, x));
}
// If f is an add expression, expand it as if it were multiplication.
if (n > 1 && f.Left is Sum)
{
Expression e = f.Left;
for (int i = 1; i < n; ++i)
{
e = Distribute(f.Left, e);
}
return(e);
}
return(f);
}
// Expand N(x)/D(x) using partial fractions.
private static Expression ExpandPartialFractions(Expression N, Expression D, Expression x)
{
List <Expression> terms = new List <Expression>();
List <Variable> unknowns = new List <Variable>();
List <Expression> basis = new List <Expression>();
foreach (Expression i in Product.TermsOf(D))
{
// Get the multiplicity of this basis term.
Expression e = i;
int n = Power.IntegralExponentOf(e);
if (n != 1)
{
e = ((Power)i).Left;
}
// Convert to a polynomial.
Polynomial Pi = Polynomial.New(e, x);
// Add new terms for each multiplicity n.
for (int j = 1; j <= n; ++j)
{
// Expression for the unknown numerator of this term.
Expression unknown = 0;
for (int k = 0; k < Pi.Degree; ++k)
{
Variable Ai = Variable.New("_A" + unknowns.Count.ToString());
unknown += Ai * (x ^ k);
unknowns.Add(Ai);
}
terms.Add(Product.New(unknown, Power.New(e, -j)));
}
basis.Add(i);
}
// Equate the original expression with the decomposed expressions.
D = Sum.New(terms.Select(j => (Expression)(D * j))).Expand();
Polynomial l = Polynomial.New(N, x);
Polynomial r = Polynomial.New(D, x);
// Equate terms of equal degree and solve for the unknowns.
int degree = Math.Max(l.Degree, r.Degree);
List <Equal> eqs = new List <Equal>(degree + 1);
for (int i = 0; i <= degree; ++i)
{
eqs.Add(Equal.New(l[i], r[i]));
}
List <Arrow> A = eqs.Solve(unknowns);
// Substitute the now knowns.
return(Sum.New(terms.Select(i => i.Evaluate(A))));
}
