The j0 functions return the value of the Bessel function of the
    first kind of order 0.

    The j1 functions return the value of the Bessel function of the
    first kind of order 1.

    The jn functions return the value of the Bessel function of the
    first kind of order n.

    The j1 and jn functions can result in an underflow as x gets
    small. The largest value of x for which this occurs is a function
    of n.