fftcausal

the question

How much does an under-padded FFT long-convolution corrupt the causal output of a sequence model?

what came out

Exactly 2L-1-N positions are wrong (aliasing identity to 3e-14), and each corrupted output depends only on future inputs, a label leak: perturbing the last input moves past position 0 by +11.8 at N=L. The next-power-of-two length is never causally safe; the safe length is double.

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