Bessel functions were used as a component of warp equations. They were used for the calculation of the energy needed for a warp jump.
In 2364, Starfleet propulsion expert Kosinski, when asked how the USS Enterprise-D flew to the Triangulum Galaxy, stated in his long-winded explanation that he may not have applied the Bessel functions correctly. In the opinion of Commander William T. Riker, the explanation was nonsensical, yet he couldn't provide an explanation for how the starship arrived at its current position. (TNG: "Where No One Has Gone Before")