2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/63415We prove that, for low-order (n < 5) stable polynomial segments or interval polynomials, there always exists a fixed polynomial such that their ratio is SPR-invariant, thereby providing a rigorous proof of Anderson's claim on SPR synthesis for the fourth-order stable interval polynomials. Moreover, the relationship between SPR synthesis for low-order polynomial segments and SPR synthesis for low-order interval polynomials is also discussed.A long-standing open problem is solved and extendedOptimization and ControlDynamical Systems93B50; 93C41text