diff --git a/Makefile.include b/Makefile.include
index fec5e89..ddb8f49 100644
--- a/Makefile.include
+++ b/Makefile.include
@@ -21,6 +21,9 @@ default: $(ALL_EXE) doc
 
 tests: run_tests.native
 
+bytelib:
+	ocamlmklib -o bytelib ./_build/Utils/Util.cmo 
+
 QCaml.odocl: $(MLIFILES)
 	ls $(MLIFILES) | sed "s/\.mli//" > QCaml.odocl
 
diff --git a/Utils/Matrix.ml b/Utils/Matrix.ml
index 4370796..2b8b620 100644
--- a/Utils/Matrix.ml
+++ b/Utils/Matrix.ml
@@ -60,11 +60,17 @@ let dense_of_sparse = function
 
 let dense_of_mat m = Dense m
 
+
 let rec to_vector_array ?(threshold=epsilon) = function
   | Sparse {m ; n ; v} -> v
   | Dense m -> to_vector_array (sparse_of_dense ~threshold (Dense m))
 
 
+let identity n = 
+ Sparse { n ; m=n ;
+   v = Array.init n (fun i -> Vector.sparse_of_assoc_list n [(i+1,1.0)])
+ }
+
 let sparse_of_mat ?(threshold=epsilon) m =
   dense_of_mat m
   |> sparse_of_dense ~threshold
@@ -268,7 +274,7 @@ let mv ?(sparse=false) ?(trans=`N) ?(threshold=epsilon) a b =
     | `T -> dim1
   in
   if f a <> Vector.dim b then
-      invalid_arg "Inconsistent dimensions";
+    invalid_arg "Inconsistent dimensions";
 
   let spmv a b =
     let {m ; n ; v} = 
@@ -290,8 +296,8 @@ let mv ?(sparse=false) ?(trans=`N) ?(threshold=epsilon) a b =
       | `T -> (fun i -> Mat.col a i)
     in
     Vec.init (Mat.dim1 a) (fun i ->
-      Vector.dense_of_vec (f_a i)
-      |> Vector.dot b )
+        Vector.dense_of_vec (f_a i)
+        |> Vector.dot b )
   in
 
   let dense_result =
@@ -307,18 +313,87 @@ let mv ?(sparse=false) ?(trans=`N) ?(threshold=epsilon) a b =
   else
     Vector.dense_of_vec  dense_result
 
-let iterative_ax_eq_b ~trans a b = 
-  failwith "Not implemented"
+
+let rec op2 dense_op sparse_op a b =
+  if dim1 a <> dim1 b || dim2 a <> dim2 b then
+    failwith "Inconsistent dimensions";
+
+  match a, b with
+  | (Dense  a), (Dense  b) -> Dense (dense_op a b)
+  | (Dense  _), (Sparse _) -> op2 dense_op sparse_op (sparse_of_dense a) b
+  | (Sparse _), (Dense  _) -> op2 dense_op sparse_op a (sparse_of_dense b)
+  | (Sparse a), (Sparse b) -> Sparse
+    { m=a.m ; n=a.n ;
+      v = Array.map2 sparse_op a.v b.v
+    }
+
+let add = op2 (fun a b -> Mat.add a b) (fun a b -> Vector.add a b)
+let sub = op2 (fun a b -> Mat.sub a b) (fun a b -> Vector.sub a b)
+
+let scale f = function
+| Dense  a -> let b = lacpy a in (Mat.scal f b ; Dense b)
+| Sparse a -> Sparse
+    { a with
+      v = if f = 1.0 then a.v
+        else Array.map (fun v -> Vector.scale f v) a.v }
+
+let frobenius_norm = function
+| Dense  a -> lange ~norm:`F a
+| Sparse a -> 
+  Array.fold_left (fun accu v -> accu +. Vector.dot v v) 0. a.v
+  |> sqrt
+
+
+
+let ax_eq_b_conj_grad ?x a b =
+  (* /!\ : A needs to be positive definite and symmetric *)
+  let x = 
+    match x with
+    | Some x0 -> x0
+    | None -> b
+  in
+  let r = Vector.sub b (mv a x) in
+  let p = r in
+  let rsold = Vector.dot r r in
+  let rec aux rsold r p x = function
+  | 0 -> x
+  | i ->
+      let ap = mv a p in
+      let alpha = rsold /. (Vector.dot p ap) in
+      let x = Vector.add x (Vector.scale alpha p) in
+      let r = Vector.sub r (Vector.scale alpha ap) in
+      let rsnew = Vector.dot r r in
+      if rsnew < Constants.epsilon then
+        x
+      else
+        let p =
+          Vector.add r (Vector.scale (rsnew /. (rsold +. 1.e-12) ) p)
+        in
+        aux rsnew r p x (i-1)
+  in
+  aux rsold r p x (Vector.dim b *2)
+
+
 
 let rec ax_eq_b ?(trans=`N) a b =
   match a, b with 
   | (Dense  a), (Dense  b) ->
-      let x = lacpy a in
-      (getrs ~trans x b; Dense x)
+    let a = lacpy a in
+    let x = lacpy b in
+    (getrs ~trans a x; Dense x)
   | (Dense  _), (Sparse _) -> 
-      let b = dense_of_sparse b in
-      ax_eq_b ~trans a b
-  | _ -> iterative_ax_eq_b ~trans a b
+    let b = dense_of_sparse b in
+    ax_eq_b ~trans a b
+  | _ ->
+    let ata, atb =
+      if trans = `N then
+        mm ~transa:`T a a, mm ~transa:`T a b 
+      else
+        mm ~transa:`N a a, mm ~transa:`N a b 
+    in
+    Sparse { m=dim1 b ; n=dim2 b ;
+      v=Array.map (fun v -> ax_eq_b_conj_grad ata v) (to_vector_array atb)
+    }
 
 
 
@@ -409,6 +484,25 @@ let test_case () =
     Alcotest.(check (float 1.e-10)) "sparse sparse" 0. (norm_diff m1_s (outer_product v1_s v2_s));
   in
 
+  let test_add_sub () =
+    let x2 = Mat.map (fun x -> if abs_float x < 0.3 then 0. else x) (Mat.random d1 d2) in
+    let m2 = dense_of_mat x2 in
+    let m3 = Mat.add x1 x2 |> dense_of_mat in
+    let m4 = Mat.sub x1 x2 |> dense_of_mat in
+    let m2_s = sparse_of_mat x2 in
+    let m3_s = Mat.add x1 x2 |> sparse_of_mat in
+    let m4_s = Mat.sub x1 x2 |> sparse_of_mat in
+    Alcotest.(check (float 1.e-10)) "dense  dense   1" 0. (norm_diff (add m1 m2) m3);
+    Alcotest.(check (float 1.e-10)) "dense  dense   2" 0. (norm_diff (sub m1 m2) m4);
+    Alcotest.(check (float 1.e-10)) "dense  sparse  3" 0. (norm_diff (add m1 m2_s) m3_s);
+    Alcotest.(check (float 1.e-10)) "dense  sparse  4" 0. (norm_diff (sub m1 m2_s) m4_s);
+    Alcotest.(check (float 1.e-10)) "sparse dense   5" 0. (norm_diff (add m1_s m2) m3);
+    Alcotest.(check (float 1.e-10)) "sparse dense   6" 0. (norm_diff (sub m1_s m2) m4);
+    Alcotest.(check (float 1.e-10)) "dense  sparse  7" 0. (norm_diff (add m1_s m2_s) m3_s);
+    Alcotest.(check (float 1.e-10)) "dense  sparse  8" 0. (norm_diff (sub m1_s m2_s) m4_s);
+    Alcotest.(check (float 1.e-10)) "dense  sparse  9" (frobenius_norm m1_s) (frobenius_norm m1);
+  in
+
   let test_mv () =
     let y    = Vec.random d2 in
     let z    = Vec.random d1 in
@@ -466,12 +560,45 @@ let test_case () =
     Alcotest.(check (float 1.e-10)) "sparse sparse 15" 0. (norm_diff (mm ~transb:`T m1_s m5_s) m3_s);
     Alcotest.(check (float 1.e-10)) "sparse sparse 16" 0. (norm_diff (transpose (mm m2_s m1_s ~transa:`T ~transb:`T)) m3_s);
   in
+
+  let test_solve () = 
+    let x1 = Mat.map (fun x -> if abs_float x < 0.6 then 0. else x) (Mat.random 30 30)
+    and x2 = Mat.map (fun x -> if abs_float x < 0.3 then 0. else x) (Mat.random 30 5)
+    in
+
+    let m1 = dense_of_mat x1
+    and m2 = dense_of_mat x2
+    in
+
+    let m1_s = sparse_of_mat x1
+    and m2_s = sparse_of_mat x2
+    in
+
+    let a = m1 and b = m2 in
+    let x = ax_eq_b a b in
+    Alcotest.(check (float 1.e-10)) "dense dense 1" 0. (norm_diff (mm a x) b);
+
+    let a = m1 and b = m2_s in
+    let x = ax_eq_b a b in
+    Alcotest.(check (float 1.e-10)) "dense dense 2" 0. (norm_diff (mm a x) b);
+
+    let a = m1_s and b = m2 in
+    let x = ax_eq_b a b in
+    Alcotest.(check (float 1.e-10)) "dense dense 2" 0. (norm_diff (mm a x) b);
+
+    let a = m1_s and b = m2_s in
+    let x = ax_eq_b a b in
+    Alcotest.(check (float 1.e-10)) "dense dense 2" 0. (norm_diff (mm a x) b);
+  in
+
   [
     "Conversion", `Quick, test_conversion;
     "Dimensions", `Quick, test_dimensions;
     "Transposition", `Quick, test_transpose;
     "Outer product", `Quick, test_outer;
+    "Add sub", `Quick, test_add_sub;
     "Matrix Vector", `Quick, test_mv;
     "Matrix Matrix", `Quick, test_mm;
+    "Linear solve",  `Quick, test_solve;
   ]
 
diff --git a/Utils/Range.ml b/Utils/Range.ml
index b924e35..e20c240 100644
--- a/Utils/Range.ml
+++ b/Utils/Range.ml
@@ -15,7 +15,7 @@ let expand_range r =
           | i     -> i::(do_work (i+1))
         in do_work start
       end
-  | r :: [] -> int_of_string r
+  | r :: [] -> [int_of_string r]
   | [] -> []
   | _ -> invalid_arg "Only one range expected"
 
@@ -27,7 +27,7 @@ let of_string s =
     assert (s.[0] = '[') ;
     assert (s.[(String.length s)-1] = ']') ;
     let s = String.sub s 1 ((String.length s) - 2) in
-    let l = String_ext.split ~on:',' s in
+    let l = String.split_on_char ',' s in
     let l = List.map expand_range l in
   List.concat l
   |> List.sort_uniq compare
@@ -41,6 +41,6 @@ let to_string l =
 
 
 let pp_range ppf t =
-  Format.fprintf "@[%s@]" ppf (to_string t)
+  Format.fprintf ppf "@[%s@]" (to_string t)
 
 
diff --git a/Utils/Util.ml b/Utils/Util.ml
index 7672584..c1e37b0 100644
--- a/Utils/Util.ml
+++ b/Utils/Util.ml
@@ -385,7 +385,7 @@ let string_of_matrix m =
   Format.asprintf "%a" pp_matrix m
 
 let debug_matrix name a =
-  Format.printf "@[%s =\n@[%a@]@]@ " name pp_matrix a
+  Format.printf "@[%s =\n@[%a@]@]@." name pp_matrix a
 
 
 let matrix_of_file filename =
diff --git a/Utils/Vector.ml b/Utils/Vector.ml
index 0eacd52..fe1fd55 100644
--- a/Utils/Vector.ml
+++ b/Utils/Vector.ml
@@ -231,6 +231,7 @@ let dot v v' =
   | (Sparse v), (Dense  v') -> d_sp v' v
 
 
+let norm v = sqrt @@ dot v v
 
 let test_case () =
 
diff --git a/Utils/Vector.mli b/Utils/Vector.mli
index 7a7b40a..304a906 100644
--- a/Utils/Vector.mli
+++ b/Utils/Vector.mli
@@ -65,6 +65,8 @@ val axpy : ?threshold:float -> ?alpha:float -> t -> t -> t
 val dot : t -> t -> float
 (** Dot product. *)
 
+val norm : t -> float
+(** l2-norm of the vector : {% $\sqrt{\sum_i x_i^2}$ %} *)
 
 (** {1 Printers } *)