+A = tridiagonal(0, -1, 2, -1);
+expected = [];
+unit_test_equals("poisson tridiagonal of size zero is correct", ...
+ expected, ...
+ A);
+
+A = tridiagonal(1, -1, 2, -1);
+expected = [2];
+unit_test_equals("poisson tridiagonal of size one is correct", ...
+ expected, ...
+ A);
+
+A = tridiagonal(2, -1, 2, -1);
+expected = [2, -1; ...
+ -1, 2];
+unit_test_equals("poisson tridiagonal of size two is correct", ...
+ expected, ...
+ A);
+
+A = tridiagonal(3, -1, 2, -1);
+expected = [2, -1, 0; ...
+ -1, 2, -1;
+ 0, -1, 2];
+unit_test_equals("poisson tridiagonal of size three is correct", ...
+ expected, ...
+ A);