Reference:

A. Cruz, J. Esparza, and J. Sesma. Zeros of the hankel function of real order out of the principal riemann sheet. Journal of Computational und Applied Mathematics, 37:89–99, 1991.

Suggested BibTeX entry:

@article{CES91,
    author = {A. Cruz and J. Esparza and J. Sesma},
    journal = {Journal of Computational und Applied Mathematics},
    pages = {89--99},
    title = {Zeros of the Hankel function of real order out of the principal Riemann sheet},
    volume = {37},
    year = {1991}
}