Structure preserved by actual harmonic block addition #
The carrier-retaining sum is the actual HarmonicWaveInteraction.addBlock.
Smoothness of its input coefficient functions justifies linearity of the
Fréchet derivatives in the cylindrical divergence. No output divergence
condition, nonzero frequency, or nonzero angular frequency is assumed.
The complex single Fourier fields add before any real projection or differentiation. This identity also holds at harmonic zero.
Additivity uses actual differentiability of all components at the evaluation point. The geometric direction fields need no derivatives.
The carrier-retaining sum preserves modewise solenoidality when the second input is interpreted on the retained carrier.
Actual addition of two smooth solenoidal blocks on a common carrier preserves the same primitive solenoidality condition on the same strip.
The velocity zero-mode condition is preserved coefficientwise, as an identity of actual coefficient functions.
Pressure has the same algebraic preservation without any derivative or regularity assumptions.