Fixed issue #1998.

doc/changes/2082.bugfix

@@ -0,0 +1 @@
+Fixed bug were some matrices were wrongly found to be hermitian.

qutip/cy/spmath.pyx

@@ -600,8 +600,8 @@ def zcsr_isherm(object A not None, double tol = qset.atol):
                 col = col_index[ptr]
                 nxt = out_row_index[col]
                 out_row_index[col] += 1
-                # We tested the structure already, so we can guarantee that
-                # these two elements correspond.
+                if row != col_index[nxt]:
+                    return _zcsr_isherm_full(data, col_index, row_index, nrows, tolsq)
                 if not _conj_feq(data[ptr], data[nxt], tolsq):
                     return False
         return True

qutip/tests/test_spmath.py

@@ -312,5 +312,16 @@ def test_zcsr_isherm_compare_implicit_zero():
         assert not zcsr_isherm(base.T, tol=tol)
 
 
+def test_issue_1998():
+    tol = 1e-12
+    base = sp.csr_matrix(np.array([[1,1,0],
+                                   [0,1,1],
+                                   [1,0,1]],
+                                  dtype=np.complex128))
+    base = fast_csr_matrix((base.data, base.indices, base.indptr), base.shape)
+    assert not zcsr_isherm(base, tol=tol)
+    assert not zcsr_isherm(base.T, tol=tol)
+
+
 if __name__ == "__main__":
     run_module_suite()