X-Git-Url: http://gitweb.michael.orlitzky.com/?p=octave.git;a=blobdiff_plain;f=diffusion_matrix_sparse.m;fp=diffusion_matrix_sparse.m;h=dbe4e2a53a0c4af68a0374f7782dd9c5b7baab85;hp=0000000000000000000000000000000000000000;hb=4c1c0fdc1eab7fbe1e322ff651e9c98357a8ce15;hpb=542e0bbc0bcde79e1366d4280cc0637cd43586d1

diff --git a/diffusion_matrix_sparse.m b/diffusion_matrix_sparse.m
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+++ b/diffusion_matrix_sparse.m
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+function K = diffusion_matrix_sparse(integerN)
+  ##
+  ## A sparse representation of the matrix K in the advection-diffusion
+  ## equation. See advection_matrix.m for details.
+  ##
+
+  if (integerN < 2)
+    K = NA;
+    return
+  end
+
+  ## The negative ones directly above the diagonal.
+  top = [ [zeros(integerN-1, 1), -speye(integerN-1)]; ...
+          zeros(1, integerN)];
+
+  ## The negative ones directly below the diagonal.
+  bottom = [ [zeros(1, integerN-1); ...
+	     -speye(integerN-1)    ], zeros(integerN, 1)];
+
+  ## Combine the top and bottom.
+  K = top + bottom + 2*speye(integerN);
+
+  ## Fill in the entries in the corner.
+  K(1, integerN) = -1;
+  K(integerN, 1) = -1;
+end