gem install ruby-fftw3If you want to have NumRu::FFTW3 to use numru-narray (NumRu::NArray, seiya's 64-bit version of NArray), install it in advance and set the environmental variable NARRAY_TYPE as "numru-narray" (if bash, export NARRAY_TYPE=numru-narray). The default is to use narray (the original masa's NArray, not Numo::NArray).