NumCosmoXcorKernelIntegrandpeek_knots

Peeks the knots integrand‘s components are represented on, shared by every component (multipole) it carries, or NULL when integrand is not spline-backed.

These knots are what makes the outer $k$ integral exactly integrable, and are why NC_XCOR_METHOD_KERNEL_EXACT needs no tolerance. Each component is a cubic spline in $k$, so on any interval over which both members of a pair are a single cubic piece, the product $k^2 W_i(k) W_j(k)$ entering $C_\ell$ is a polynomial of degree $8$ and a $5$-node Gauss-Legendre rule integrates it exactly.

Two kernels are sampled independently and so do not share an abscissa: the intervals with that property are the panels of the common refinement of the two knot sets. Merging two sorted knot vectors is all the coupling the argument needs — sampling the kernels jointly onto one shared abscissa would also work but costs about twice as much to produce, and forces every pair to carry every kernel’s knots.