diff --git a/src/kernel.rs b/src/kernel.rs
index 0ea844c..29ebbaf 100644
--- a/src/kernel.rs
+++ b/src/kernel.rs
@@ -35,18 +35,8 @@ pub trait Kernel {
     fn measurement_vector(&self) -> Array1<f64>;
     fn feedback(&self) -> Array2<f64>;
     fn transition(&self, t0: f64, t1: f64) -> Array2<f64>;
-
-    fn noise_effect(&self) -> Array2<f64> {
-        unimplemented!();
-    }
-
-    fn noise_density(&self) -> Array2<f64> {
-        unimplemented!();
-    }
-
-    fn noise_cov(&self, _t0: f64, _t1: f64) -> Array2<f64> {
-        unimplemented!();
-    }
+    fn noise_effect(&self) -> Array2<f64>;
+    fn noise_cov(&self, t0: f64, t1: f64) -> Array2<f64>;
 }
 
 impl Kernel for Vec<Box<dyn Kernel>> {