Scilab Function
Last update : 24/02/2006

oldbesseli - Modified Bessel functions of the first kind (I sub alpha).

oldbesselj - Bessel functions of the first kind (J sub alpha).

oldbesselk - Modified Bessel functions of the second kind (K sub alpha).

oldbessely - Bessel functions of the second kind (Y sub alpha).

### Calling Sequence

  `y = oldbesseli(alpha,x)`
  `y = oldbesseli(alpha,x,ice)`
  `y = oldbesselj(alpha,x)`
  `y = oldbesselk(alpha,x)`
  `y = oldbesselk(alpha,x,ice)`
  `y = oldbessely(alpha,x)`

### Parameters

- `x`: real vector with non negative entries
- `alpha`: real vector with non negative entries regularly spaced with increment equal to one `alpha=alpha0+(n1:n2)`
- `ice`: integer flag, with default value 1

### Description

These functions are obsolete, use `besseli`, `besselj`, `besselk`, `bessely` instead. Note however that the semantics of these two sets of functions are different.

`oldbesseli(alpha,x)` computes modified Bessel functions of the first kind (I sub alpha), for real, non-negative order `alpha` and real non negative argument `x`. `besseli(alpha,x,2)` computes `besseli(alpha,x).*exp(-x)`.

`oldbesselj(alpha,x)` computes Bessel functions of the first kind (J sub alpha), for real, non-negative order `alpha` and real non negative argument `x`.

`oldbesselk(alpha,x)` computes modified Bessel functions of the second kind (K sub alpha), for real, non-negative order `alpha` and real non negative argument `x`. `besselk(alpha,x,2)` computes `besselk(alpha,x).*exp(x)`.

`oldbessely(alpha,x)` computes Bessel functions of the second kind (Y sub alpha), for real, non-negative order `alpha` and real non negative argument `x`.

`alpha` and `x` may be vectors. The output is `m`-by-`n` with `m = size(x,'*')`, `n = size(alpha,'*')` whose `(i,j)` entry is `oldbessel?(alpha(j),x(i))`.

### Remarks

Y_alpha and J_alpha Bessel functions are 2 independant solutions of the Bessel 's differential equation :

    2 2 2 x y" + x y' + (x - alpha ) y = 0 , alpha >=
        0

K_alpha and I_alpha modified Bessel functions are 2 independant solutions of the modified Bessel 's differential equation :

    2 2 2 x y" + x y' - (x + alpha ) y = 0 , alpha >=
        0

### Examples

    // example #1: display some I Bessel functions
    x = linspace(0.01,10,5000)';
    y = oldbesseli(0:4,x);
    ys = oldbesseli(0:4,x,2);
    xbasc()
    subplot(2,1,1)
       plot2d(x,y, style=2:6, leg="I0@I1@I2@I3@I4", rect=[0,0,6,10])
       xtitle("Some modified Bessel functions of the first kind")
    subplot(2,1,2)
       plot2d(x,ys, style=2:6, leg="I0s@I1s@I2s@I3s@I4s", rect=[0,0,6,1])
       xtitle("Some modified scaled Bessel functions of the first kind")

    // example #2 : display some J Bessel functions
    x = linspace(0,40,5000)';
    y = besselj(0:4,x);
    xbasc()
    plot2d(x,y, style=2:6, leg="J0@J1@J2@J3@J4")
    xtitle("Some Bessel functions of the first kind")

    // example #3 : use the fact that J_(1/2)(x) = sqrt(2/(x pi)) sin(x)
    //              to compare the algorithm of besselj(0.5,x) with
    //              a more direct formula
    x = linspace(0.1,40,5000)';
    y1 = besselj(0.5, x);
    y2 = sqrt(2 ./(%pi*x)).*sin(x);
    er = abs((y1-y2)./y2);
    ind = find(er > 0 & y2 ~= 0);
    xbasc()
    subplot(2,1,1)
       plot2d(x,y1,style=2)
       xtitle("besselj(0.5,x)")
    subplot(2,1,2)
       plot2d(x(ind), er(ind), style=2, logflag="nl")
       xtitle("relative error between 2 formulae for besselj(0.5,x)")

    // example #4: display some K Bessel functions
    x = linspace(0.01,10,5000)';
    y = besselk(0:4,x);
    ys = besselk(0:4,x,1);
    xbasc()
    subplot(2,1,1)
       plot2d(x,y, style=0:4, leg="K0@K1@K2@K3@K4", rect=[0,0,6,10])
       xtitle("Some modified Bessel functions of the second kind")
    subplot(2,1,2)
       plot2d(x,ys, style=0:4, leg="K0s@K1s@K2s@K3s@K4s", rect=[0,0,6,10])
       xtitle("Some modified scaled Bessel functions of the second kind")

    // example #5: plot severals Y Bessel functions
    x = linspace(0.1,40,5000)'; // Y Bessel functions are unbounded  for x -> 0+
    y = bessely(0:4,x);
    xbasc()
    plot2d(x,y, style=0:4, leg="Y0@Y1@Y2@Y3@Y4", rect=[0,-1.5,40,0.6])
    xtitle("Some Bessel functions of the second kind")

### Author

W. J. Cody, L. Stoltz (code from Netlib (specfun))