Undocumented g0(x) and g1(x)



To my surprise, the functions g0() and g1() are defined as

(C1) g0(x);

                                              - x
(D1)                         BESSEL_I(0, X) %E
(C2) g1(x);

                                              - x
(D2)                         BESSEL_I(1, X) %E


I can't find any documentation on this.