X-Git-Url: http://gitweb.michael.orlitzky.com/?p=octave.git;a=blobdiff_plain;f=advection_matrix_sparse.m;fp=advection_matrix_sparse.m;h=ce45e159d84f0d28510240ee08c145833ca8ce78;hp=0000000000000000000000000000000000000000;hb=9b1a3c694305734969d771a6e67a67123006b5ca;hpb=b39006334c16dcd7256cb7c36e88996229863e6b

diff --git a/advection_matrix_sparse.m b/advection_matrix_sparse.m
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+function S = advection_matrix_sparse(integerN)
+  ##
+  ## Sparse version of the advection_matrix function. See
+  ## advection_matrix.m for details.
+  ##
+
+  if (integerN < 2)
+    S = NA;
+    return
+  end
+
+  ## The ones directly above the diagonal.
+  top = [ [zeros(integerN-1, 1), speye(integerN-1)]; ...
+          zeros(1, integerN)];
+
+  ## The ones directly below the diagonal.
+  bottom = [ [zeros(1, integerN-1); ...
+	     -speye(integerN-1)    ], zeros(integerN, 1)];
+
+  ## Combine the top and bottom.
+  S = top + bottom;
+
+  ## Fill in the entries in the corner.
+  S(1, integerN) = -1;
+  S(integerN, 1) = 1;
+
+  ## And divide the whole thing by 2.
+  S = (1/2)*S;
+end