Robust SPR synthesis for low-order polynomial segments and interval polynomials

We prove that, for low-order (n ⩽ 4) stable polynomial segments or interval polynomials, there always exists a fixed polynomial such that their ratio is strict positive realness (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