Exploits the conjugate symmetry, i.e. f{l, -m} = (-1)^m f^*{l, m}, to avoid computing the coefficients for negative m, as they are just conjugate to the positive values. Consequently this reduces both the number of computations and the amount of memory required by ~ a factor of 2. See this line in S2FFT where we generate an explicitly real signal, from which hopefully its clear what is meant by conjugate symmetry
Exploits the conjugate symmetry, i.e. f{l, -m} = (-1)^m f^*{l, m}, to avoid computing the coefficients for negative m, as they are just conjugate to the positive values. Consequently this reduces both the number of computations and the amount of memory required by ~ a factor of 2. See this line in S2FFT where we generate an explicitly real signal, from which hopefully its clear what is meant by conjugate symmetry