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# 23. Numerical

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## 23.1 Introduction to Numerical

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## 23.2 Fourier packages

The `fft` package comprises functions for the numerical (not symbolic) computation of the fast Fourier transform. `load ("fft")` loads this package. See `fft`.

The `fourie` package comprises functions for the symbolic computation of Fourier series. `load ("fourie")` loads this package. There are functions in the `fourie` package to calculate Fourier integral coefficients and some functions for manipulation of expressions. See `Definitions for Fourier Series`.

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## 23.3 Definitions for Numerical

Function: polartorect (magnitude_array, phase_array)

  Translates complex values of the form `r %e^(%i t)` to the form `a + b %i`. `load ("fft")` loads this function into Maxima. See also `fft`.

  The magnitude and phase, `r` and `t`, are taken from magnitude_array and phase_array, respectively. The original values of the input arrays are replaced by the real and imaginary parts, `a` and `b`, on return. The outputs are calculated as

      a: r cos (t)
      b: r sin (t)

  The input arrays must be the same size and 1-dimensional. The array size need not be a power of 2.

  `polartorect` is the inverse function of `recttopolar`.

Function: recttopolar (real_array, imaginary_array)

  Translates complex values of the form `a + b %i` to the form `r %e^(%i t)`. `load ("fft")` loads this function into Maxima. See also `fft`.

  The real and imaginary parts, `a` and `b`, are taken from real_array and imaginary_array, respectively. The original values of the input arrays are replaced by the magnitude and angle, `r` and `t`, on return. The outputs are calculated as

      r: sqrt (a^2 + b^2)
      t: atan2 (b, a)

  The computed angle is in the range `-%pi` to `%pi`.

  The input arrays must be the same size and 1-dimensional. The array size need not be a power of 2.

  `recttopolar` is the inverse function of `polartorect`.

Function: ift (real_array, imaginary_array)

  Fast inverse discrete Fourier transform. `load ("fft")` loads this function into Maxima.

  `ift` carries out the inverse complex fast Fourier transform on 1-dimensional floating point arrays. The inverse transform is defined as

      x[j]: sum (y[j] exp (+2 %i %pi j k / n), k, 0, n-1)

  See `fft` for more details.

Function: fft (real_array, imaginary_array)
Function: ift (real_array, imaginary_array)
Function: recttopolar (real_array, imaginary_array)
Function: polartorect (magnitude_array, phase_array)

  Fast Fourier transform and related functions. `load ("fft")` loads these functions into Maxima.

  `fft` and `ift` carry out the complex fast Fourier transform and inverse transform, respectively, on 1-dimensional floating point arrays. The size of imaginary_array must equal the size of real_array.

  `fft` and `ift` operate in-place. That is, on return from `fft` or `ift`, the original content of the input arrays is replaced by the output. The `fillarray` function can make a copy of an array, should it be necessary.

  The discrete Fourier transform and inverse transform are defined as follows. Let `x` be the original data, with

      x[i]: real_array[i] + %i imaginary_array[i]

  Let `y` be the transformed data. The forward and inverse transforms are

      y[k]: (1/n) sum (x[j] exp (-2 %i %pi j k / n), j, 0, n-1)

      x[j]:       sum (y[j] exp (+2 %i %pi j k / n), k, 0, n-1)

  Suitable arrays can be allocated by the `array` function. For example:

      array (my_array, float, n-1)$

  declares a 1-dimensional array with n elements, indexed from 0 through n-1 inclusive. The number of elements n must be equal to 2^m for some m.

  `fft` can be applied to real data (imaginary array all zeros) to obtain sine and cosine coefficients. After calling `fft`, the sine and cosine coefficients, say `a` and `b`, can be calculated as

      a[0]: real_array[0]
      b[0]: 0

  and

      a[j]: real_array[j] + real_array[n-j]
      b[j]: imaginary_array[j] - imaginary_array[n-j]

  for j equal to 1 through n/2-1, and

      a[n/2]: real_array[n/2]
      b[n/2]: 0

  `recttopolar` translates complex values of the form `a + b %i` to the form `r %e^(%i t)`. See `recttopolar`.

  `polartorect` translates complex values of the form `r %e^(%i t)` to the form `a + b %i`. See `polartorect`.

  `demo ("fft")` displays a demonstration of the `fft` package.

Option variable: fortindent

  Default value: 0

  `fortindent` controls the left margin indentation of expressions printed out by the `fortran` command. 0 gives normal printout (i.e., 6 spaces), and positive values will causes the expressions to be printed farther to the right.

Function: fortran (expr)

  Prints expr as a Fortran statement. The output line is indented with spaces. If the line is too long, `fortran` prints continuation lines. `fortran` prints the exponentiation operator `^` as `**`, and prints a complex number `a + b %i` in the form `(a,b)`.

  expr may be an equation. If so, `fortran` prints an assignment statement, assigning the right-hand side of the equation to the left-hand side. In particular, if the right-hand side of expr is the name of a matrix, then `fortran` prints an assignment statement for each element of the matrix.

  If expr is not something recognized by `fortran`, the expression is printed in `grind` format without complaint. `fortran` does not know about lists, arrays, or functions.

  `fortindent` controls the left margin of the printed lines. 0 is the normal margin (i.e., indented 6 spaces). Increasing `fortindent` causes expressions to be printed further to the right.

  When `fortspaces` is `true`, `fortran` fills out each printed line with spaces to 80 columns.

  `fortran` evaluates its arguments; quoting an argument defeats evaluation. `fortran` always returns `done`.

  Examples:

      (%i1) expr: (a + b)^12$
      (%i2) fortran (expr);
            (b+a)**12
      (%o2)                         done
      (%i3) fortran ('x=expr);
            x = (b+a)**12
      (%o3)                         done
      (%i4) fortran ('x=expand (expr));
            x = b**12+12*a*b**11+66*a**2*b**10+220*a**3*b**9+495*a**4*b**8+792
           1   *a**5*b**7+924*a**6*b**6+792*a**7*b**5+495*a**8*b**4+220*a**9*b
           2   **3+66*a**10*b**2+12*a**11*b+a**12
      (%o4)                         done
      (%i5) fortran ('x=7+5*%i);
            x = (7,5)
      (%o5)                         done
      (%i6) fortran ('x=[1,2,3,4]);
            x = [1,2,3,4]
      (%o6)                         done
      (%i7) f(x) := x^2$
      (%i8) fortran (f);
            f
      (%o8)                         done

Option variable: fortspaces

  Default value: `false`

  When `fortspaces` is `true`, `fortran` fills out each printed line with spaces to 80 columns.

Function: horner (expr, x)
Function: horner (expr)

  Returns a rearranged representation of expr as in Horner's rule, using x as the main variable if it is specified. `x` may be omitted in which case the main variable of the canonical rational expression form of expr is used.

  `horner` sometimes improves stability if `expr` is to be numerically evaluated. It is also useful if Maxima is used to generate programs to be run in Fortran. See also `stringout`.

      (%i1) expr: 1e-155*x^2 - 5.5*x + 5.2e155;
                                 2
      (%o1)            1.0E-155 x  - 5.5 x + 5.2E+155
      (%i2) expr2: horner (%, x), keepfloat: true;
      (%o2)            (1.0E-155 x - 5.5) x + 5.2E+155
      (%i3) ev (expr, x=1e155);
      Maxima encountered a Lisp error:

       floating point overflow

      Automatically continuing.
      To reenable the Lisp debugger set *debugger-hook* to nil.
      (%i4) ev (expr2, x=1e155);
      (%o4)                       7.0E+154

Function: find_root (f(x), x, a, b)
Function: find_root (f, a, b)

  Finds the zero of function f as variable x varies over the range `[a, b]`. The function must have a different sign at each endpoint. If this condition is not met, the action of the function is governed by `find_root_error`. If `find_root_error` is `true` then an error occurs, otherwise the value of `find_root_error` is returned (thus for plotting `find_root_error` might be set to 0.0). Otherwise (given that Maxima can evaluate the first argument in the specified range, and that it is continuous) `find_root` is guaranteed to come up with the zero (or one of them if there is more than one zero). The accuracy of `find_root` is governed by `find_root_abs` and `find_root_rel` which must be non-negative floating point numbers. `find_root` will stop when the first arg evaluates to something less than or equal to `find_root_abs` or if successive approximants to the root differ by no more than `find_root_rel * <one of the approximants>`. The default values of `find_root_abs` and `find_root_rel` are 0.0 so `find_root` gets as good an answer as is possible with the single precision arithmetic we have. The first arg may be an equation. The order of the last two args is irrelevant. Thus

      find_root (sin(x) = x/2, x, %pi, 0.1);

  is equivalent to

      find_root (sin(x) = x/2, x, 0.1, %pi);

  The method used is a binary search in the range specified by the last two args. When it thinks the function is close enough to being linear, it starts using linear interpolation.

  Examples:

      (%i1) f(x) := sin(x) - x/2;
                                              x
      (%o1)                  f(x) := sin(x) - -
                                              2
      (%i2) find_root (sin(x) - x/2, x, 0.1, %pi);
      (%o2)                   1.895494267033981
      (%i3) find_root (sin(x) = x/2, x, 0.1, %pi);
      (%o3)                   1.895494267033981
      (%i4) find_root (f(x), x, 0.1, %pi);
      (%o4)                   1.895494267033981
      (%i5) find_root (f, 0.1, %pi);
      (%o5)                   1.895494267033981

Option variable: find_root_abs

  Default value: 0.0

  `find_root_abs` is the accuracy of the `find_root` command is governed by `find_root_abs` and `find_root_rel` which must be non-negative floating point numbers. `find_root` will stop when the first arg evaluates to something less than or equal to `find_root_abs` or if successive approximants to the root differ by no more than `find_root_rel * <one of the approximants>`. The default values of `find_root_abs` and `find_root_rel` are 0.0 so `find_root` gets as good an answer as is possible with the single precision arithmetic we have.

Option variable: find_root_error

  Default value: `true`

  `find_root_error` governs the behavior of `find_root`. When `find_root` is called, it determines whether or not the function to be solved satisfies the condition that the values of the function at the endpoints of the interpolation interval are opposite in sign. If they are of opposite sign, the interpolation proceeds. If they are of like sign, and `find_root_error` is `true`, then an error is signaled. If they are of like sign and `find_root_error` is not `true`, the value of `find_root_error` is returned. Thus for plotting, `find_root_error` might be set to 0.0.

Option variable: find_root_rel

  Default value: 0.0

  `find_root_rel` is the accuracy of the `find_root` command is governed by `find_root_abs` and `find_root_rel` which must be non-negative floating point numbers. `find_root` will stop when the first arg evaluates to something less than or equal to `find_root_abs` or if successive approximants to the root differ by no more than `find_root_rel * <one of the approximants>`. The default values of `find_root_abs` and `find_root_rel` are 0.0 so `find_root` gets as good an answer as is possible with the single precision arithmetic we have.

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## 23.4 Definitions for Fourier Series

Function: equalp (x, y)

  Returns `true` if `equal (x, y)` otherwise `false` (doesn't give an error message like `equal (x, y)` would do in this case).

Function: remfun (f, expr)
Function: remfun (f, expr, x)

  `remfun (f, expr)` replaces all occurrences of `f (arg)` by arg in expr.

  `remfun (f, expr, x)` replaces all occurrences of `f (arg)` by arg in expr only if arg contains the variable x.

Function: funp (f, expr)
Function: funp (f, expr, x)

  `funp (f, expr)` returns `true` if expr contains the function f.

  `funp (f, expr, x)` returns `true` if expr contains the function f and the variable x is somewhere in the argument of one of the instances of f.

Function: absint (f, x, halfplane)
Function: absint (f, x)
Function: absint (f, x, a, b)

  `absint (f, x, halfplane)` returns the indefinite integral of f with respect to x in the given halfplane (`pos`, `neg`, or `both`). f may contain expressions of the form `abs (x)`, `abs (sin (x))`, `abs (a) * exp (-abs (b) * abs (x))`.

  `absint (f, x)` is equivalent to `absint (f, x, pos)`.

  `absint (f, x, a, b)` returns the definite integral of f with respect to x from a to b. f may include absolute values.

Function: fourier (f, x, p)

  Returns a list of the Fourier coefficients of `f(x)` defined on the interval `[-%pi, %pi]`.

Function: foursimp (l)

  Simplifies `sin (n %pi)` to 0 if `sinnpiflag` is `true` and `cos (n %pi)` to `(-1)^n` if `cosnpiflag` is `true`.

Option variable: sinnpiflag

  Default value: `true`

  See `foursimp`.

Option variable: cosnpiflag

  Default value: `true`

  See `foursimp`.

Function: fourexpand (l, x, p, limit)

  Constructs and returns the Fourier series from the list of Fourier coefficients l up through limit terms (limit may be `inf`). x and p have same meaning as in `fourier`.

Function: fourcos (f, x, p)

  Returns the Fourier cosine coefficients for `f(x)` defined on `[0, %pi]`.

Function: foursin (f, x, p)

  Returns the Fourier sine coefficients for `f(x)` defined on `[0, %pi]`.

Function: totalfourier (f, x, p)

  Returns `fourexpand (foursimp (fourier (f, x, p)), x, p, 'inf)`.

Function: fourint (f, x)

  Constructs and returns a list of the Fourier integral coefficients of `f(x)` defined on `[minf, inf]`.

Function: fourintcos (f, x)

  Returns the Fourier cosine integral coefficients for `f(x)` on `[0, inf]`.

Function: fourintsin (f, x)

  Returns the Fourier sine integral coefficients for `f(x)` on `[0, inf]`.

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