Man pages matching name rtf_create_fir_lowpass in section 3:

rtf_create_fir_lowpass.3.en	debian 10 / librtfilter-dev
rtf_create_fir_lowpass.3.en	debian 11 / librtfilter-dev
rtf_create_fir_lowpass.3.en	debian 12 / librtfilter-dev