Add fix for recursive verifier.

plonky2/src/fri/oracle.rs

@@ -155,6 +155,8 @@ impl<F: RichField + Extendable<D>, C: GenericConfig<D, F = F>, const D: usize>
             final_poly += quotient;
         }
         final_poly.trim();
+        // Multiply the final polynomial by `X`, so that `final_poly` has the maximum degree for
+        // which the LDT will pass. See github.com/mir-protocol/plonky2/pull/434 for details.
         final_poly.coeffs.insert(0, F::Extension::ZERO);
 
         let lde_final_poly = final_poly.lde(fri_params.config.rate_bits);

plonky2/src/fri/recursive_verifier.rs

@@ -281,7 +281,7 @@ impl<F: RichField + Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             sum = self.div_add_extension(numerator, denominator, sum);
         }
 
-        sum
+        self.mul_extension(sum, subgroup_x)
     }
 
     fn fri_verifier_query_round<C: GenericConfig<D, F = F>>(