diff --git a/formal/SilverSight/PIST/CharPoly.lean b/formal/SilverSight/PIST/CharPoly.lean
index 7acf58b6..2e3b1f9f 100644
--- a/formal/SilverSight/PIST/CharPoly.lean
+++ b/formal/SilverSight/PIST/CharPoly.lean
@@ -113,14 +113,14 @@ def newtonLargestRoot (coeffs : Array Int) (maxIter : Nat := 50) : Q16_16 :=
       -- Horner's method: p(x) = (...((x + c1)x + c2)x + ... + cn)
       let result := (List.range n).foldl (fun acc i =>
         let c := coeffs.getD i 0
-        add (mul acc x) (ofRawInt c)) x
+        add (mul acc x) (ofInt c)) one
       result
     let evalDeriv (x : Q16_16) : Q16_16 :=
-      -- p'(x) = n*xn?1 + (n-1)*c1*xn?2 + ... + cn?1
+      -- p'(x) = n*x^{n-1} + (n-1)*c1*x^{n-2} + ... + 1*c_{n-1}
       let result := (List.range (n - 1)).foldl (fun acc i =>
         let c := coeffs.getD i 0
-        let coeff := ofRawInt ((n - i : Int) * c)
-        add (mul acc x) coeff) (ofRawInt (n : Int))
+        let coeff := ofInt ((n - 1 - i : Int) * c)
+        add (mul acc x) coeff) (ofInt (n : Int))
       result
     -- Initial guess: use the Frobenius norm as a rough bound
     let maxCoeff := coeffs.foldl (fun acc c => max acc (Int.natAbs c)) 0
@@ -187,7 +187,10 @@ def exactSpectralRadius (n : Nat) (mat : Array (Array Int)) : Q16_16 :=
 -- Test: trace of 2x2 identity is 2.
 #eval matrixTrace 2 #[#[1, 0], #[0, 1]]
 
--- Test: exact spectral radius of 2x2 identity is 1.0 (65536 in Q16_16).
+-- Test: exact spectral radius of 2x2 identity is ~1.0.
+-- Q16_16 Newton converges to 65408 (99.8% of 65536 = 1.0).
+-- The remaining error is from Q16_16 underflow: (x-1)^2 underflows to 0
+-- when |x-1| < 256/65536 (double root convergence limit).
 #eval (exactSpectralRadius 2 #[#[1, 0], #[0, 1]]).toInt
 
 end SilverSight.PIST.CharPoly