Department of Mathematics, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.|Department of Pharmacology, School of Dentistry, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hokkaido 061-0293, Japan.|Department of Mathematics, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.|Department of Mathematics, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.