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+function test_unusual_input(verbose, plot)
+
+% This function is used to test whether the function diff_matrices1d will
+% create an output for D1xb, D1xc, D1xf, or D1xx when the inputs for 
+% N and dx don't fit the task.
+
+% It will return 'true' if diff_matrices1d:
+% - doesn't generate any of the 4 matrices when at least one of the inputs is 'incorrect';
+% - generates the 4 matrices when the inputs are 'correct'.
+% It will return 'false' if diff_matrices1d:
+% - generates at least one of the 4 matrices even though at least one of the inputs is 'incorrect';
+% - doesn't generate the 4 matrices when the inputs are 'correct'.
+
+% Here we take the specific example of:
+% N = 3; (correct)
+% dx = nan; (incorrect)
+
+% It turns out that when dx = nan, outputs is returned with enties NAN.
+
+if nargin == 0
+    verbose = false;
+    plot = false;
+elseif nargin == 1
+    plot = false;
+end
+
+N = 3;                   % Those inputs could be adapted for other (N,dx)
+dx = nan;
+
+try                     
+[Ix,D1xx,D1xc,D1xb,D1xf] = diff_matrices1d(N,dx,'n');        % the last argument can be chosen between BC, 'n', or 'd'
+end 
+
+if (~(rem(N,1)==0)) | (~(isnumeric(dx))) | (dx == Inf) | (isnan(dx))
+   r = (exist('D1xb') == 0)&(exist('D1xc') == 0)&(exist('D1xf') == 0)&(exist('D1xx') == 0);
+else
+   r = (exist('D1xb') == 1)&(exist('D1xc') == 1)&(exist('D1xf') == 1)&(exist('D1xx') == 1);
+end
+
+% return true/false if the test passed/failed
+if r == 0
+    disp ('false')
+elseif r == 1
+    disp ('true')
+end 
+end