/// <summary>
/// Computes the square root of a complex number.
/// </summary>
/// <param name="z">The argument.</param>
/// <returns>The square root of the argument.</returns>
/// <remarks>
/// <para>The image below shows the complex square root function near the origin, using domain coloring.</para>
/// <img src="../images/ComplexSqrtPlot.png" />
/// <para>You can see the branch cut extending along the negative real axis from the zero at the origin.</para>
/// </remarks>
public static Complex Sqrt(Complex z)
{
if (z.Im == 0.0)
{
// Handle the degenerate case quickly.
// This also eliminates need to worry about Im(z) = 0 in subsequent formulas.
if (z.Re < 0.0)
{
return(new Complex(0.0, Math.Sqrt(-z.Re)));
}
else
{
return(new Complex(Math.Sqrt(z.Re), 0.0));
}
}
else
{
// To find a fast formula for complex square root, note x + i y = \sqrt{a + i b} implies
// x^2 + 2 i x y - y^2 = a + i b, so x^2 - y^2 = a and 2 x y = b. Cross-substitute and
// use quadratic formula to solve for x^2 and y^2 to obtain
// x = \sqrt{\frac{\sqrt{a^2 + b^2} + a}{2}} y = \pm \sqrt{\frac{\sqrt{a^2 + b^2} - a}{2}}
// This gives complex square root in three square roots and a few flops.
// Only problem is b << a case, where significant cancelation occurs in one of the formula.
// (Which one depends on the sign of a.) Handle that case by series expansion.
double p, q;
if (Math.Abs(z.Im) < 0.25 * Math.Abs(z.Re))
{
double x2 = MoreMath.Sqr(z.Im / z.Re);
double t = x2 / 2.0;
double s = t;
// Find s = \sqrt{1 + x^2} - 1 using binomial expansion
for (int k = 2; true; k++)
{
if (k > Global.SeriesMax)
{
throw new NonconvergenceException();
}
double s_old = s;
t *= (1.5 / k - 1.0) * x2;
s += t;
if (s == s_old)
{
break;
}
}
if (z.Re < 0.0)
{
p = -z.Re * s;
q = -2.0 * z.Re + p;
}
else
{
q = z.Re * s;
p = 2.0 * z.Re + q;
}
}
else
{
double m = ComplexMath.Abs(z);
p = m + z.Re;
q = m - z.Re;
}
double x = Math.Sqrt(p / 2.0);
double y = Math.Sqrt(q / 2.0);
if (z.Im < 0.0)
{
y = -y;
}
return(new Complex(x, y));
/*
* if (Math.Abs(z.Im) < 0.125 * Math.Abs(z.Re)) {
* // We should try to improve this by using a series instead of the full power algorithm.
* return (Pow(z, 0.5));
* } else {
* // This is a pretty fast formula for a complex square root, basically just
* // three square roots and a few flops.
* // But if z.Im << z.Re, then z.Re ~ m and it suffers from cancelations.
* double m = Abs(z);
* double x = Math.Sqrt((m + z.Re) / 2.0);
* double y = Math.Sqrt((m - z.Re) / 2.0);
* if (z.Im < 0.0) y = -y;
* return (new Complex(x, y));
* }
*/
}
}
/// <summary>
/// Raises a complex number to an integer power.
/// </summary>
/// <param name="z">The argument.</param>
/// <param name="n">The power.</param>
/// <returns>The value of z<sup>n</sup>.</returns>
public static Complex Pow(Complex z, int n)
{
// this is a straight-up copy of MoreMath.Pow with x -> z, double -> Complex
if (n < 0)
{
return(1.0 / Pow(z, -n));
}
switch (n)
{
case 0:
// we follow convention that 0^0 = 1
return(1.0);
case 1:
return(z);
case 2:
// 1 multiply
return(z * z);
case 3:
// 2 multiplies
return(z * z * z);
case 4: {
// 2 multiplies
Complex z2 = z * z;
return(z2 * z2);
}
case 5: {
// 3 multiplies
Complex z2 = z * z;
return(z2 * z2 * z);
}
case 6: {
// 3 multiplies
Complex z2 = z * z;
return(z2 * z2 * z2);
}
case 7: {
// 4 multiplies
Complex z3 = z * z * z;
return(z3 * z3 * z);
}
case 8: {
// 3 multiplies
Complex z2 = z * z;
Complex z4 = z2 * z2;
return(z4 * z4);
}
case 9: {
// 4 multiplies
Complex z3 = z * z * z;
return(z3 * z3 * z3);
}
case 10: {
// 4 multiplies
Complex z2 = z * z;
Complex z4 = z2 * z2;
return(z4 * z4 * z2);
}
default:
return(ComplexMath.Pow(z, (double)n));
}
}
/// <summary>
/// Raises a complex number to an integer power.
/// </summary>
/// <param name="z">The argument.</param>
/// <param name="n">The power.</param>
/// <returns>The value of z<sup>n</sup>.</returns>
public static Complex Pow(Complex z, int n)
{
if (n < 0)
{
return(1.0 / Pow(z, -n));
}
switch (n)
{
case 0:
// We follow convention that 0^0 = 1
return(1.0);
case 1:
return(z);
case 2:
// 1 multiply
return(Sqr(z));
case 3:
// 2 multiplies
return(Sqr(z) * z);
case 4: {
// 2 multiplies
Complex z2 = Sqr(z);
return(Sqr(z2));
}
case 5: {
// 3 multiplies
Complex z2 = Sqr(z);
return(Sqr(z2) * z);
}
case 6: {
// 3 multiplies
Complex z2 = Sqr(z);
return(Sqr(z2) * z2);
}
case 7: {
// 4 multiplies
Complex z3 = Sqr(z) * z;
return(Sqr(z3) * z);
}
case 8: {
// 3 multiplies
Complex z2 = Sqr(z);
Complex z4 = Sqr(z2);
return(Sqr(z4));
}
case 9: {
// 4 multiplies
Complex z3 = Sqr(z) * z;
return(Sqr(z3) * z3);
}
case 10: {
// 4 multiplies
Complex z2 = Sqr(z);
Complex z4 = Sqr(z2);
return(Sqr(z4) * z2);
}
case 12: {
// 4 multiplies
Complex z3 = Sqr(z) * z;
Complex z6 = Sqr(z3);
return(Sqr(z6));
}
case 16: {
// 4 multiplies
Complex z2 = Sqr(z);
Complex z4 = Sqr(z2);
Complex z8 = Sqr(z4);
return(Sqr(z8));
}
// that's all the cases do-able in 4 or fewer complex multiplies
default:
return(ComplexMath.Pow(z, (double)n));
}
}
