Mostafa Blidia Pierre Duchet Henry Jacob Frédéric Maffray Henry Meyniel Some operations preserving the existence of kernels. 211-216 1999 205 Discrete Mathematics 1-3 http://dx.doi.org/10.1016/S0012-365X(99)00026-6 db/journals/dm/dm205.html#BlidiaDJMM99