funcion.bas

ZBRENT rutine for VB6

Attribute VB_Name = "modFuncion"
Option Explicit

Public Function fx(ByVal x As Double) As Double
    fx = bessj0(x)
End Function

Public Function func(ByVal x As Double) As Double
    func = bessj0(x)
End Function

Public Function bessj0(ByVal x As Double) As Double

    Dim ax As Double, z As Double, aux1 As Double, aux2 As Double
    Dim xx As Double, y As Double, ans As Double, ans1 As Double, ans2 As Double

    ax = Abs(x)
    If ax < 8# Then
        y = x * x
        aux1 = -11214424.18 + y * (77392.33017 + y * (-184.9052456))
        ans1 = 57568490574# + y * (-13362590354# + y * (651619640.7 + y * aux1))
        aux2 = 59272.64853 + y * (267.8532712 + y * 1#)
        ans2 = 57568490411# + y * (1029532985# + y * (9494680.718 + y * aux2))
        ans = ans1 / ans2
    Else
        z = 8# / ax
        y = z * z
        xx = ax - 0.785398164
        aux1 = -0.000002073370639 + y * 2.093887211E-07
        ans1 = 1# + y * (-0.001098628627 + y * (0.00002734510407 + y * aux1))
        aux2 = -0.000006911147651 + y * (7.621095161E-07 - y * 9.34945152E-08)
        ans2 = -0.01562499995 + y * (0.0001430488765 + y * aux2)
        ans = Sqr(0.636619772 / ax) * (Cos(xx) * ans1 - z * Sin(xx) * ans2)
    End If
    bessj0 = ans
End Function