Philosophical Transactions of the Royal Society of London. Series A. Mathematical, Physical and Engineering Sciences (Philos. Trans. R. Soc. Lond. Ser. A Math. Phys. Eng. Sci.) (20110101), 369, no.~1945, 2467-2475. ISSN: 1364-503X (print).eISSN: 1471-2962.