QENS fit functions - jump diffusion

#pragma rtGlobals=1     // Use modern global access method.
//Jobic, H. & Theodorou, D.
//"Quasi-elastic neutron scattering and molecular dynamics simulation
// as complementary techniques for studying diffusion in zeolites",
// Microporous and Mesoporous Materials, 2007, 102, 21-50

//note that we are fitting FWHM, NOT HWHM, so there are factors of two difference.
Function chudleyelliot(w, y, x):fitfunc
    wave w, y, x
    //eqn 51
    //w[0] = tau
    //w[1] = d

    y = sin(x * w[1]) / (x * w[1])
    y = 1 - y
    y /= w[0] / 2
End

Function hallross(w, y, x):fitfunc
    wave w, y, x
    //eqn 53
    //w[0] = tau
    //w[1] = <r^2>

    y = exp(-1 * x^2 * w[1] / 6)
    y = 1 - y
    y /= w[0] / 2
End

Function singwisjolander(w, y, x):fitfunc
    wave w, y, x
    //eqn 55
    //w[0] = tau
    //w[1] = <r^2>

    y = x^2 * w[1]
    y /= 1 + x^2 * w[1] / 6
    y /= w[0] * 6 / 2
End