Linear spline reconstruction. The primordial power spectrum is reconstructed using N_knots interpolation points : i = 1,2,...,N_knots }. The end knots are fixed in k but allowed to vary in , whereas the internal knots can vary subject to the constraint that k₁<k₂< ··· <k_Nknots. The function is constructed within the range [ k₁,k_Nknots ] by interpolating logarithmically between adjacent knots (i.e., linearly in log -log  space). Outside this range the function is extrapolated logarithmically. The function thus has 2N_knots−2 parameters.