Differentiating through Conjugate Gradient

This is the pre-print version of an article published by Taylor & Francis in Optimization Methods and Software on 6 January 2018, available online at: https://doi.org/10.1080/10556788.2018.1425862.We show that, although the Conjugate Gradient (CG) Algorithm has a singularity at the solution, it is possible to differentiate forward through the algorithm automatically by re-declaring all the variables as truncated Taylor series, the type of active variable widely used in Automatic Differentiation (AD) tools such as ADOL-C. If exact arithmetic is used, this approach gives a complete sequence of correct directional derivatives of the solution, to arbitrary order, in a single cycle of at most n iterations, where n is the number of dimensions. In the inexact case the approach emphasizes the need for a means by which the programmer can communicate certain conditions involving derivative values directly to an AD tool.Peer reviewe

A Cloud-based RFID Authentication Protocol with Insecure Communication Channels

© 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Radio Frequency Identification (RFID) has becomea widespread technology to automatically identify objects and withthe development of cloud computing, cloud-based RFID systemsattract more research these days. Several cloud-based RFIDauthentication protocols have been proposed to address privacyand security properties in the environment where the cloudprovider is untrusted therefore the tag’s data are encrypted andanonymously stored in the cloud database. However, most of thecloud-based RFID authentication protocols assume securecommunication channels between the reader and the cloud server.To protect data transmission between the reader and the cloudserver without any help from a third party, this paper proposes acloud-based RFID authentication protocol with insecurecommunication channels (cloud-RAPIC) between the reader and the cloud server. The cloud-RAPIC protocol preserves tag privacyeven when the tag does not update its identification. The cloudRAPIC protocol has been analyzed using the UPriv model andAVISPA verification tool which have proved that the protocolpreserves tag privacy and protects data secrecy

The Academic Dress of Doctors of Philosophy at the University of London

Of the sixteen UK universities that instituted the PhD degree by 1920, of which London was the last, only London, Wales, and Birmingham specify a doctor’s full-dress pattern robe in a darker shade of red than scarlet. The warm red shade used for PhD robes at Wales and Birmingham has always been called ‘crimson‘ there, but at London the Medici crimson used was called ‘claret’ for reasons that remain obscure. Today the cloth used for PhD robes at London is considerably darker than crimson, almost a maroon, but Dr Isabel Soar’s robes show that this was not the case in 1920. It remains for London’s PhD graduates to prevail upon robemakers to restore their robes to the original bright warm crimson.Peer reviewe

A Purple Passion? Queen’s College Oxford and the Blood of the Lord 63

The Queen’s College Oxford was founded in 1341 ‘under the name of the Hall of the Queen’s scholars of Oxford\u27 by the endowment of Robert de Eglesfield. The queen in question was Queen Philippa of Hainault, consort of King Edward III of England: Robert Eglesfield, who became Provost of the college, was her chaplain. The college statutes contain one tantalising passage that might or might not refer to something that we would regard as academic dress, which is contained in and discussed in the article. [Excerpt]

Oxford Blues: The Search for the Origins of the Lay Bachelors’ Hood

One particular anomaly of the Oxford system, alluded to several times by Franklyn, is the use of blue silk hoods lined with fur for bachelors’ degrees in Civil Law and in Medicine (BCL and BM). How did two such different degrees come to be given the same hood? Why blue, which features nowhere in the corresponding doctors’ hoods? And why is the blue on the outside, instead of inside where a silk lining belongs? In this short paper, we examine these three questions and propose some hypotheses for further investigation. [Excerpt]

A Portrait of an Oxford Nobleman, circa 1705

Peer reviewe

Reflections on Designing the Academic Dress of the University of Hertfordshire

© 2021 The Authors. This is an open access article distributed under the Creative Commons Non Commercial Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by-nc/4.0/Thirty years ago the authors were involved in the design of the academic dress for the new University of Hertfordshire. Here we reflect upon the process, and try to remem- ber what we thought we were doing.Peer reviewe

Cheap Newton steps for optimal control problems: automatic differentiation and Pantoja's algorithm

Original article can be found at: http://www.informaworld.com/smpp/title~content=t713645924~db=all Copyright Taylor and Francis / Informa.In this paper we discuss Pantoja's construction of the Newton direction for discrete time optimal control problems. We show that automatic differentiation (AD) techniques can be used to calculate the Newton direction accurately, without requiring extensive re-writing of user code, and at a surprisingly low computational cost: for an N-step problem with p control variables and q state variables at each step, the worst case cost is 6(p + q + 1) times the computational cost of a single target function evaluation, independent of N, together with at most p3/3 + p2(q + 1) + 2p(q + 1)2 + (q + l)3, i.e. less than (p + q + l)3, floating point multiply-and-add operations per time step. These costs may be considerably reduced if there is significant structural sparsity in the problem dynamics. The systematic use of checkpointing roughly doubles the operation counts, but reduces the total space cost to the order of 4pN floating point stores. A naive approach to finding the Newton step would require the solution of an Np Np system of equations together with a number of function evaluations proportional to Np, so this approach to Pantoja's construction is extremely attractive, especially if q is very small relative to N. Straightforward modifications of the AD algorithms proposed here can be used to implement other discrete time optimal control solution techniques, such as differential dynamic programming (DDP), which use state-control feedback. The same techniques also can be used to determine with certainty, at the cost of a single Newton direction calculation, whether or not the Hessian of the target function is sufficiently positive definite at a point of interest. This allows computationally cheap post-hoc verification that a second-order minimum has been reached to a given accuracy, regardless of what method has been used to obtain it.Peer reviewe

Simulating Perceptions of Security

This document is the Accepted Manuscript of a conference paper published in Proceedings of the Cambridge International Workshop on Security Protocols 2017. Under embargo until 29 November 2018. The final publication is available at Springer via: https://doi.org/10.1007/978-3-319-71075-4_7Systems complicated enough to have ongoing security issues are difficult to understand, and hard to model. The models are hard to understand, even when they are right (another reason they are usually wrong), and too complicated to use to make decisions. Instead attackers, developers, and users make security decisions based on their {\em perceptions} of the system, and not on properties that the system actually has. These perceptions differ between communities, causing decisions made by one community to appear irrational to another. Attempting to predict such irrational behaviour by basing a model of perception on a model of the system is even more complicated than the original modelling problem we can't solve. Ockham's razor says to model the perceptions directly, since these will be simpler than the system itself.Final Accepted Versio