The Fast Legendre Transformation employed in this package carries out a FLT by means of a DCT. This was first described by Alpert & Rokhlin (1991):
Alpert, B. K., & Rokhlin, V., 1991, SIAM Journal on Scientific and Statistical Computing, 12, 158. doi:10.1137/0912009
The specific implementation transforms between DCT and FLT using a recurrence, without explicitly constructing the matrix of Alpert & Rokhlin (1991). For more information, see:
Tessore N., Loureiro A., Joachimi B., von Wietersheim-Kramsta M., Jeffrey N., 2023, OJAp, 6, 11. doi:10.21105/astro.2302.01942