File path = /ww/webwork/pg/lib/Complex.pm

**

• NAME
• SYNOPSIS
• DESCRIPTION
• OPERATIONS
• CREATION
• STRINGIFICATION
• USAGE
• ERRORS DUE TO DIVISION BY ZERO OR LOGARITHM OF ZERO
• ERRORS DUE TO INDIGESTIBLE ARGUMENTS
• BUGS
• AUTHORS

NAME

Math::Complex - complex numbers and associated mathematical functions

SYNOPSIS

    use Math::Complex;

    $z = Math::Complex->make(5, 6);
    $t = 4 - 3*i + $z;
    $j = cplxe(1, 2*pi/3);

DESCRIPTION

This package lets you create and manipulate complex numbers. By default, Perl limits itself to real numbers, but an extra use statement brings full complex support, along with a full set of mathematical functions typically associated with and/or extended to complex numbers.

If you wonder what complex numbers are, they were invented to be able to solve the following equation:

    x*x = -1

and by definition, the solution is noted i (engineers use j instead since i usually denotes an intensity, but the name does not matter). The number i is a pure imaginary number.

The arithmetics with pure imaginary numbers works just like you would expect it with real numbers... you just have to remember that

    i*i = -1

so you have:

    5i + 7i = i * (5 + 7) = 12i
    4i - 3i = i * (4 - 3) = i
    4i * 2i = -8
    6i / 2i = 3
    1 / i = -i

Complex numbers are numbers that have both a real part and an imaginary part, and are usually noted:

    a + bi

where a is the real part and b is the imaginary part. The arithmetic with complex numbers is straightforward. You have to keep track of the real and the imaginary parts, but otherwise the rules used for real numbers just apply:

    (4 + 3i) + (5 - 2i) = (4 + 5) + i(3 - 2) = 9 + i
    (2 + i) * (4 - i) = 2*4 + 4i -2i -i*i = 8 + 2i + 1 = 9 + 2i

A graphical representation of complex numbers is possible in a plane (also called the complex plane, but it's really a 2D plane). The number

    z = a + bi

is the point whose coordinates are (a, b). Actually, it would be the vector originating from (0, 0) to (a, b). It follows that the addition of two complex numbers is a vectorial addition.

Since there is a bijection between a point in the 2D plane and a complex number (i.e. the mapping is unique and reciprocal), a complex number can also be uniquely identified with polar coordinates:

    [rho, theta]

where rho is the distance to the origin, and theta the angle between the vector and the x axis. There is a notation for this using the exponential form, which is:

    rho * exp(i * theta)

where i is the famous imaginary number introduced above. Conversion between this form and the cartesian form a + bi is immediate:

    a = rho * cos(theta)
    b = rho * sin(theta)

which is also expressed by this formula:

    z = rho * exp(i * theta) = rho * (cos theta + i * sin theta)

In other words, it's the projection of the vector onto the x and y axes. Mathematicians call rho the norm or modulus and theta the argument of the complex number. The norm of z will be noted abs(z).

The polar notation (also known as the trigonometric representation) is much more handy for performing multiplications and divisions of complex numbers, whilst the cartesian notation is better suited for additions and subtractions. Real numbers are on the x axis, and therefore theta is zero or pi.

All the common operations that can be performed on a real number have been defined to work on complex numbers as well, and are merely extensions of the operations defined on real numbers. This means they keep their natural meaning when there is no imaginary part, provided the number is within their definition set.

For instance, the sqrt routine which computes the square root of its argument is only defined for non-negative real numbers and yields a non-negative real number (it is an application from R+ to R+). If we allow it to return a complex number, then it can be extended to negative real numbers to become an application from R to C (the set of complex numbers):

    sqrt(x) = x >= 0 ? sqrt(x) : sqrt(-x)*i

It can also be extended to be an application from C to C, whilst its restriction to R behaves as defined above by using the following definition:

    sqrt(z = [r,t]) = sqrt(r) * exp(i * t/2)

Indeed, a negative real number can be noted [x,pi] (the modulus x is always non-negative, so [x,pi] is really -x, a negative number) and the above definition states that

    sqrt([x,pi]) = sqrt(x) * exp(i*pi/2) = [sqrt(x),pi/2] = sqrt(x)*i

which is exactly what we had defined for negative real numbers above. The sqrt returns only one of the solutions: if you want the both, use the root function.

All the common mathematical functions defined on real numbers that are extended to complex numbers share that same property of working as usual when the imaginary part is zero (otherwise, it would not be called an extension, would it?).

A new operation possible on a complex number that is the identity for real numbers is called the conjugate, and is noted with an horizontal bar above the number, or ~z here.

        z = a + bi
    ~z = a - bi

Simple... Now look:

    z * ~z = (a + bi) * (a - bi) = a*a + b*b

We saw that the norm of z was noted abs(z) and was defined as the distance to the origin, also known as:

    rho = abs(z) = sqrt(a*a + b*b)

so

    z * ~z = abs(z) ** 2

If z is a pure real number (i.e. b == 0), then the above yields:

    a * a = abs(a) ** 2

which is true (abs has the regular meaning for real number, i.e. stands for the absolute value). This example explains why the norm of z is noted abs(z): it extends the abs function to complex numbers, yet is the regular abs we know when the complex number actually has no imaginary part... This justifies a posteriori our use of the abs notation for the norm.

OPERATIONS

Given the following notations:

    z1 = a + bi = r1 * exp(i * t1)
    z2 = c + di = r2 * exp(i * t2)
    z = <any complex or real number>

the following (overloaded) operations are supported on complex numbers:

    z1 + z2 = (a + c) + i(b + d)
    z1 - z2 = (a - c) + i(b - d)
    z1 * z2 = (r1 * r2) * exp(i * (t1 + t2))
    z1 / z2 = (r1 / r2) * exp(i * (t1 - t2))
    z1 ** z2 = exp(z2 * log z1)
    ~z = a - bi
    abs(z) = r1 = sqrt(a*a + b*b)
    sqrt(z) = sqrt(r1) * exp(i * t/2)
    exp(z) = exp(a) * exp(i * b)
    log(z) = log(r1) + i*t
    sin(z) = 1/2i (exp(i * z1) - exp(-i * z))
    cos(z) = 1/2 (exp(i * z1) + exp(-i * z))
    atan2(z1, z2) = atan(z1/z2)

The following extra operations are supported on both real and complex numbers:

    Re(z) = a
    Im(z) = b
    arg(z) = t
    abs(z) = r

    cbrt(z) = z ** (1/3)
    log10(z) = log(z) / log(10)
    logn(z, n) = log(z) / log(n)

    tan(z) = sin(z) / cos(z)

    csc(z) = 1 / sin(z)
    sec(z) = 1 / cos(z)
    cot(z) = 1 / tan(z)

    asin(z) = -i * log(i*z + sqrt(1-z*z))
    acos(z) = -i * log(z + i*sqrt(1-z*z))
    atan(z) = i/2 * log((i+z) / (i-z))

    acsc(z) = asin(1 / z)
    asec(z) = acos(1 / z)
    acot(z) = atan(1 / z) = -i/2 * log((i+z) / (z-i))

    sinh(z) = 1/2 (exp(z) - exp(-z))
    cosh(z) = 1/2 (exp(z) + exp(-z))
    tanh(z) = sinh(z) / cosh(z) = (exp(z) - exp(-z)) / (exp(z) + exp(-z))

    csch(z) = 1 / sinh(z)
    sech(z) = 1 / cosh(z)
    coth(z) = 1 / tanh(z)

    asinh(z) = log(z + sqrt(z*z+1))
    acosh(z) = log(z + sqrt(z*z-1))
    atanh(z) = 1/2 * log((1+z) / (1-z))

    acsch(z) = asinh(1 / z)
    asech(z) = acosh(1 / z)
    acoth(z) = atanh(1 / z) = 1/2 * log((1+z) / (z-1))

arg, abs, log, csc, cot, acsc, acot, csch, coth, acosech, acotanh, have aliases rho, theta, ln, cosec, cotan, acosec, acotan, cosech, cotanh, acosech, acotanh, respectively. Re, Im, arg, abs, rho, and theta can be used also also mutators. The cbrt returns only one of the solutions: if you want all three, use the root function.

The root function is available to compute all the n roots of some complex, where n is a strictly positive integer. There are exactly n such roots, returned as a list. Getting the number mathematicians call j such that:

    1 + j + j*j = 0;

is a simple matter of writing:

    $j = ((root(1, 3))[1];

The kth root for z = [r,t] is given by:

    (root(z, n))[k] = r**(1/n) * exp(i * (t + 2*k*pi)/n)

The spaceship comparison operator, <=>, is also defined. In order to ensure its restriction to real numbers is conform to what you would expect, the comparison is run on the real part of the complex number first, and imaginary parts are compared only when the real parts match.

CREATION

To create a complex number, use either:

    $z = Math::Complex->make(3, 4);
    $z = cplx(3, 4);

if you know the cartesian form of the number, or

    $z = 3 + 4*i;

if you like. To create a number using the polar form, use either:

    $z = Math::Complex->emake(5, pi/3);
    $x = cplxe(5, pi/3);

instead. The first argument is the modulus, the second is the angle (in radians, the full circle is 2*pi). (Mnemonic: e is used as a notation for complex numbers in the polar form).

It is possible to write:

    $x = cplxe(-3, pi/4);

but that will be silently converted into [3,-3pi/4], since the modulus must be non-negative (it represents the distance to the origin in the complex plane).

It is also possible to have a complex number as either argument of either the make or emake: the appropriate component of the argument will be used.

    $z1 = cplx(-2,  1);
    $z2 = cplx($z1, 4);

STRINGIFICATION

When printed, a complex number is usually shown under its cartesian form a+bi, but there are legitimate cases where the polar format [r,t] is more appropriate.

By calling the routine Complex1::display_format and supplying either "polar" or "cartesian", you override the default display format, which is "cartesian". Not supplying any argument returns the current setting.

This default can be overridden on a per-number basis by calling the display_format method instead. As before, not supplying any argument returns the current display format for this number. Otherwise whatever you specify will be the new display format for this particular number.

For instance:

    use Math::Complex;

    Complex1::display_format('polar');
    $j = ((root(1, 3))[1];
    print "j = $j\n";       # Prints "j = [1,2pi/3]
    $j->display_format('cartesian');
    print "j = $j\n";       # Prints "j = -0.5+0.866025403784439i"

The polar format attempts to emphasize arguments like k*pi/n (where n is a positive integer and k an integer within [-9,+9]).

USAGE

Thanks to overloading, the handling of arithmetics with complex numbers is simple and almost transparent.

Here are some examples:

    use Math::Complex;

    $j = cplxe(1, 2*pi/3);  # $j ** 3 == 1
    print "j = $j, j**3 = ", $j ** 3, "\n";
    print "1 + j + j**2 = ", 1 + $j + $j**2, "\n";

    $z = -16 + 0*i;         # Force it to be a complex
    print "sqrt($z) = ", sqrt($z), "\n";

    $k = exp(i * 2*pi/3);
    print "$j - $k = ", $j - $k, "\n";

    $z->Re(3);          # Re, Im, arg, abs,
    $j->arg(2);         # (the last two aka rho, theta)
                    # can be used also as mutators.

ERRORS DUE TO DIVISION BY ZERO OR LOGARITHM OF ZERO

The division (/) and the following functions

    log ln  log10   logn
    tan sec csc cot
    atan    asec    acsc    acot
    tanh    sech    csch    coth
    atanh   asech   acsch   acoth

cannot be computed for all arguments because that would mean dividing by zero or taking logarithm of zero. These situations cause fatal runtime errors looking like this

    cot(0): Division by zero.
    (Because in the definition of cot(0), the divisor sin(0) is 0)
    Died at ...

or

    atanh(-1): Logarithm of zero.
    Died at...

For the csc, cot, asec, acsc, acot, csch, coth, asech, acsch, the argument cannot be 0 (zero). For the the logarithmic functions and the atanh, acoth, the argument cannot be 1 (one). For the atanh, acoth, the argument cannot be -1 (minus one). For the atan, acot, the argument cannot be i (the imaginary unit). For the atan, acoth, the argument cannot be -i (the negative imaginary unit). For the tan, sec, tanh, the argument cannot be pi/2 + k * pi, where k is any integer.

Note that because we are operating on approximations of real numbers, these errors can happen when merely `too close' to the singularities listed above. For example tan(2*atan2(1,1)+1e-15) will die of division by zero.

ERRORS DUE TO INDIGESTIBLE ARGUMENTS

The make and emake accept both real and complex arguments. When they cannot recognize the arguments they will die with error messages like the following

    Complex1::make: Cannot take real part of ...
    Complex1::make: Cannot take real part of ...
    Complex1::emake: Cannot take rho of ...
    Complex1::emake: Cannot take theta of ...

BUGS

Saying use Math::Complex; exports many mathematical routines in the caller environment and even overrides some (sqrt, log). This is construed as a feature by the Authors, actually... ;-)

All routines expect to be given real or complex numbers. Don't attempt to use BigFloat, since Perl has currently no rule to disambiguate a '+' operation (for instance) between two overloaded entities.

In Cray UNICOS there is some strange numerical instability that results in root(), cos(), sin(), cosh(), sinh(), losing accuracy fast. Beware. The bug may be in UNICOS math libs, in UNICOS C compiler, in Math::Complex. Whatever it is, it does not manifest itself anywhere else where Perl runs.

AUTHORS

Raphael Manfredi <Raphael_Manfredi@grenoble.hp.com> and Jarkko Hietaniemi <jhi@iki.fi>.

Extensive patches by Daniel S. Lewart <d-lewart@uiuc.edu>.

File path = /ww/webwork/pg/lib/Complex1.pm

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