Genealogical Expansions of Positive Fredholm Operators via a Reference-Point Method
We study positive Fredholm integral operators that arise as next-generation operators in structured population models. The main problem is to represent the dominant eigenvalue and the associated right and left eigenfunctions without using Fredholm determinants or finite-dimensional discretization. We introduce a reference-point construction: a rank-one correction on the space of kernels, determined by a fixed pair \((x_0,y_0)\), which reorganizes iterated kernels into a renewal-type series. Under an explicit dominant spectral separation assumption and a scalar non-resonance condition for the chosen reference pair, the resulting \(Γ_n\)-series converges at the spectral radius pointwise absolutely and gives the leading eigensystem. The coefficients also have a closed combinatorial expression in terms of ordinary partial Bell polynomials. For discrete-time integral projection models and for multi-state McKendrick equations, the same construction yields Euler--Lotka-type characteristic equations and formulas for demographic quantities such as stable distributions, reproductive values, type reproduction numbers, generation intervals, and expected generation numbers. The resulting genealogical expansion resolves the leading eigensystem into successive reproductive and transition contributions encoded by the iterated kernels.