n1</code> by <code>n0</code> in two dimensions).  This provides a
significant performance advantage, because it means that the final
transposition step can be omitted.  FFTW supports this optimization,
which you specify by passing the flag <code>FFTW_MPI_TRANSPOSED_OUT</code>
to the planner routines.  To compute the inverse transform of
transposed output, you specify <code>FFTW_MPI_TRANSPOSED_IN</code> to tell
it that the input is transposed.  In this section, we explain how to
interpret the output format o