--- a/+sbp/+implementations/d4_variable_6_2.m	Mon Sep 26 08:57:25 2016 +0200
+++ b/+sbp/+implementations/d4_variable_6_2.m	Mon Sep 26 09:04:57 2016 +0200
@@ -20,56 +20,43 @@
     % Denna ?r noggrannare, och har 2a ordningens randdslutning och b?r ge 6te
     % ordningens konvergens. Hade dock ingen fri parameter att optimera
 
+    BP = 6;
+    if(m<2*BP)
+        error(['Operator requires at least ' num2str(2*BP) ' grid points']);
+    end
 
-    H = diag(ones(m,1),0);
-    H(1:6,1:6) = [
-        0.181e3/0.576e3 0 0 0 0 0;
-        0 0.1343e4/0.960e3 0 0 0 0;
-        0 0 0.293e3/0.480e3 0 0 0;
-        0 0 0 0.1811e4/0.1440e4 0 0;
-        0 0 0 0 0.289e3/0.320e3 0;
-        0 0 0 0 0 0.65e2/0.64e2;
-    ];
-    H(m-5:m,m-5:m) = fliplr(flipud(H(1:6,1:6)));
+    % Norm
+    Hv = ones(m,1);
+    Hv(1:6) = [0.181e3/0.576e3, 0.1343e4/0.960e3, 0.293e3/0.480e3, 0.1811e4/0.1440e4, 0.289e3/0.320e3, 0.65e2/0.64e2];
+    Hv(m-5:m) = rot90(Hv(1:6),2);
+    Hv = h*Hv;
+    H = spdiag(Hv, 0);
+    HI = spdiag(1./Hv, 0);
 
 
-    e_1 = zeros(m,1);e_1(1) = 1;
-    e_m = zeros(m,1);e_m(m) = 1;
+    % Boundary operators
+    e_l = sparse(m,1);
+    e_l(1) = 1;
+    e_r = rot90(e_l, 2);
 
-    S_U = [-0.137e3/0.60e2 5 -5 0.10e2/0.3e1 -0.5e1/0.4e1 0.1e1/0.5e1;]/h;
-    S_1 = zeros(1,m);
-    S_1(1:6) = S_U;
-    S_m = zeros(1,m);
-    S_m(m-5:m) = fliplr(-S_U);
+    d1_l = sparse(m,1);
+    d1_l(1:6) = [-0.137e3/0.60e2 5 -5 0.10e2/0.3e1 -0.5e1/0.4e1 0.1e1/0.5e1;]/h;
+    d1_r = -rot90(d1_l);
 
-    S2_U = [0.15e2/0.4e1 -0.77e2/0.6e1 0.107e3/0.6e1 -13 0.61e2/0.12e2 -0.5e1/0.6e1;]/h^2;
-    S2_1 = zeros(1,m);
-    S2_1(1:6) = S2_U;
-    S2_m = zeros(1,m);
-    S2_m(m-5:m) = fliplr(S2_U);
+    d2_l = sparse(m,1);
+    d2_l(1:6) = [0.15e2/0.4e1 -0.77e2/0.6e1 0.107e3/0.6e1 -13 0.61e2/0.12e2 -0.5e1/0.6e1;]/h^2;
+    d2_r = rot90(d2_l, 2);
 
-    S3_U = [-0.17e2/0.4e1 0.71e2/0.4e1 -0.59e2/0.2e1 0.49e2/0.2e1 -0.41e2/0.4e1 0.7e1/0.4e1;]/h^3;
-    S3_1 = zeros(1,m);
-    S3_1(1:6) = S3_U;
-    S3_m = zeros(1,m);
-    S3_m(m-5:m) = fliplr(-S3_U);
+    d3_l = sparse(m,1);
+    d3_l(1:6) = [-0.17e2/0.4e1 0.71e2/0.4e1 -0.59e2/0.2e1 0.49e2/0.2e1 -0.41e2/0.4e1 0.7e1/0.4e1;]/h^3;
+    d3_r = -rot90(d3_l, 2);
 
 
-    H = h*H;
-    HI = inv(H);
-
     % Fourth derivative, 1th order accurate at first 8 boundary points (still
     % yield 5th order convergence if stable: for example u_tt = -u_xxxx
-
-    m4 = 7/240;
-    m3 = -2/5;
-    m2 = 169/60;
-    m1 = -122/15;
-    m0 = 91/8;
-
-    M4 = m4*(diag(ones(m-4,1),4)+diag(ones(m-4,1),-4))+m3*(diag(ones(m-3,1),3)+diag(ones(m-3,1),-3))+m2*(diag(ones(m-2,1),2)+diag(ones(m-2,1),-2))+m1*(diag(ones(m-1,1),1)+diag(ones(m-1,1),-1))+m0*diag(ones(m,1),0);
-
-    %M4 = (-1/6*(diag(ones(m-3,1),3)+diag(ones(m-3,1),-3) ) + 2*(diag(ones(m-2,1),2)+diag(ones(m-2,1),-2)) -13/2*(diag(ones(m-1,1),1)+diag(ones(m-1,1),-1)) + 28/3*diag(ones(m,1),0));
+    stencil = [7/240, -2/5, 169/60, -122/15, 91/8, -122/15, 169/60, -2/5, 7/240];
+    diags = -4:4;
+    M4 = stripeMatrix(stencil, diags, m);
 
     M4_U = [
         0.1009e4/0.192e3 -0.7657e4/0.480e3 0.9307e4/0.480e3 -0.509e3/0.40e2 0.4621e4/0.960e3 -0.25e2/0.32e2;
@@ -80,10 +67,10 @@
         -0.25e2/0.32e2 0.2657e4/0.960e3 -0.521e3/0.120e3 0.559e3/0.96e2 -0.1499e4/0.160e3 0.2225e4/0.192e3;
     ];
 
-    M4(1:6,1:6) = M4_U;
 
-    M4(m-5:m,m-5:m) = flipud( fliplr( M4_U ) );
-    M4 = M4/h^3;
+    M4(1:6,1:6) = M4_U;
+    M4(m-5:m,m-5:m) = rot90(M4_U, 2);
+    M4 = 1/h^3*M4;
 
-    D4 = HI*(M4-e_1*S3_1+e_m*S3_m  + S_1'*S2_1-S_m'*S2_m);
+    D4=HI*(M4 - e_l*d3_l'+e_r*d3_r' + d1_l*d2_l'-d1_r*d2_r');
 end