num::spectral::debug::verify_parseval

Defined in header <spectral/debug.hpp>

template <class Plan>
requires transform_plan<Plan>
void verify_parseval(const Plan &plan, real tol = 1e-10,
                     std::source_location loc = std::source_location::current());

Verify Parseval's identity for a forward transform.

\[ \sum_n |x_n|^2 = \frac{1}{N} \sum_k |X_k|^2 \]

A transform that violates it is not unitary after normalization, and a spectral method built on it will gain or lose energy every step.