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POFFTW: Inverse of forward fft not equal to original function
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  1. POFFTW: Inverse of forward fft not equal to original function
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    copied!<p>I'm trying to use FFTW to compute fast summations, and I've run into an issue: </p> <pre><code>int numFreq3 = numFreq*numFreq*numFreq; FFTW_complex* dummy_sq_fft = (FFTW_complex*)FFTW_malloc( sizeof(FFTW_complex)*numFreq3 ); FFTW_complex* dummy_sq = (FFTW_complex*)FFTW_malloc( sizeof(FFTW_complex)*numFreq3 ); FFTW_complex* orig = (FFTW_complex*)FFTW_malloc( sizeof(FFTW_complex)*numFreq3 ); FFTW_plan dummyPlan = FFTW_plan_dft_3d( numFreq, numFreq, numFreq, orig, dummy_sq_fft, FFTW_FORWARD, FFTW_MEASURE ); FFTW_plan dummyInvPlan = FFTW_plan_dft_3d( numFreq, numFreq, numFreq, dummy_sq_fft, dummy_sq, FFTW_BACKWARD, FFTW_MEASURE ); for(int i= 0; i &lt; numFreq3; i++) { orig[ i ][ 0 ] = sparseProfile02[ 0 ][ i ][ 0 ]; //img. part == 0 orig[ i ][ 1 ] = sparseProfile02[ 0 ][ i ] [ 1 ]; } FFTW_execute(dummyPlan); FFTW_execute(dummyInvPlan); int count = 0; for(int i=0; i&lt;numFreq3; i++) { double factor = dummy_sq[ i ][ 0 ]/sparseProfile02[ 0 ][ i ][ 0 ]; if(factor &lt; 0) { count++; } } std::cout&lt;&lt;"Count "&lt;&lt;count&lt;&lt;"\n"; FFTW_free(dummy_sq_fft); FFTW_free(dummy_sq); FFTW_destroy_plan(dummyPlan); FFTW_destroy_plan(dummyInvPlan); </code></pre> <p>(Here sparseProfile02[0] is of type FFTW_complex*, and contains only positive real data.)</p> <p>Since we have dummy_sq = IFFT(FFT(sparseProfile02[ 0 ])), we must have dummy_sq = n^3*sparseProfile02. But this is true only some of the time; in fact, values on the dummy_sq grid are negative whenever corresponding values on the sparseProfile02 grid are zero (but not vice-versa). Does anyone know why this is happening?</p>
 

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