Carbocations as Lewis Acid Catalysts: Reactivity and Scope

One class of potential Lewis acids that has received negligible attention as a catalyst is the carbocation. Here we show the potential of triarylmethylium ions as highly powerful Lewis acid catalysts for organic reactions. The Lewis acidity of the triarylmethylium ion can be easily tuned by variation of the electronic properties of the aromatic rings and the catalytic activity of the carbocation is shown to correlate directly to the level of stabilization of the empty p_C‐orbital at the cationic carbon. The versatility of triarylmethylium ions as efficient Lewis acid catalysts for organic reactions is demonstrated in Diels–Alder, aza‐Diels–Alder, conjugate addition, halogenation, epoxide rearrangement and intramolecular hetro‐ene reactions.

     

An Elliptic Curve Trapdoor System

We propose an elliptic curve trapdoor system which is of interest in key escrow applications. In this system, a pair (E_s, E_pb) of elliptic curves over F₂¹⁶¹ is constructed with the following properties: (i) the Gaudry-Hess-Smart Weil descent attack reduces the elliptic curve discrete logarithm problem (ECDLP) in E_s(F₂¹⁶¹) to a hyperelliptic curve DLP in the Jacobian of a curve of genus 7 or 8, which is computationally feasible, but by far not trivial; (ii) E_s is isogenous to E_s; (iii) the best attack on the ECDLP in E_s(F₂¹⁶¹) is the parallelized Pollard rho method. The curve E_s is used just as usual in elliptic curve cryptosystems. The curve E_s is submitted to a trusted authority for the purpose of key escrow. The crucial difference from other key escrow scenarios is that the trusted authority has to invest a considerable amount of computation to compromise a user's private key, which makes applications such as widespread wire-tapping impossible.     

On the complexity of drawing trees nicely: corrigendum

In this journal, Supowit and Reingold [1] have given a proof that it is NP-complete to decide whether a binary tree can be drawn on a grid with width 24 if certain æsthetic requirements are obeyed. We identify and repair a mistake in their proof.Received: 19 November 2003, Published online: 21 April 2004     

Two-Fraction Electron System on a Thin Helium Film

A systematic theoretical investigation of microwave absorption of 2-dimensional electron systems above a thin helium film in the presence of a cyclotron resonance magnetic field is presented. To explain the measured data, a two-fraction structure of the electron system is introduced. One component corresponds to the free electron motion, the second one takes into account electron localization near the potential minimum caused by the roughness of the substrate. Within this model the general dependence of microwave absorption becomes understandable. The details of the observed cyclotron resonance line-shift are discussed.     

Analysis and suppression of high-frequency inverter modulation in sensorless position-controlled induction machine drives

This paper focuses on the modulation of high-frequency (HF) currents required for sensorless position control of induction machine drives. Such drives can be realized by using the modulating effect of the rotor slots or from an engineered rotor saliency. Distortion due to saturation is known to cause a deterioration in the rotor position estimate. This paper reports a hitherto unreported distortion arising from inverter deadtime that will affect all sensorless drives using HF injection techniques. The deadtime distortion behaves in a similar way to that of saturation but has some specific characteristics that are presented in this work. Standard deadtime compensation techniques reduce but do not eliminate the distorting modulation. The residual distortion should be suppressed for good position estimation. The paper presents a new compensation strategy termed space modulation profiling which is effective in suppressing the HF modulations due to inverter deadtime and saturation saliencies. Experimental results show the sensorless drive under heavy load for zero-speed and transient operation.     

Results of transsphenoidal hypophysectomy in 52 dogs with pituitary-dependent hyperadrenocorticism

OBJECTIVE: To evaluate microsurgical transsphenoidal hypophysectomy in dogs with pituitary-dependent hyperadrenocorticism (PDH). STUDY DESIGN: Prospective study to evaluate the results (survival and disease-free interval, remission, recurrence) and complications of microsurgical transsphenoidal hypophysectomy by clinical follow-up, computed tomography (CT), and urinary corticoid-to-creatinine (C/C) ratios in dogs with PDH. The effect of surgical experience was investigated by comparing results of hypophysectomy cases 1 through 26 and 27 through 52. ANIMALS OR SAMPLE POPULATION: 52 dogs with PDH. RESULTS: Preoperative CT enabled accurate assessment of pituitary size (24 nonenlarged and 28 enlarged) and localization relative to intraoperative anatomic landmarks. Treatment failures included procedure-related mortalities (five dogs) and incomplete hypophysectomies (four dogs). The 1-year estimated survival rate was 84% (95% confidence interval [CI], 71% to 92%). The 2-year estimated survival rate was 80% (95% CI, 65% to 90%). In 43 dogs, the hyperadrenocorticism went into remission. Hyperadrenocorticism recurred in five dogs. The 1-year estimated relapse-free fraction was 92% (95% CI, 76% to 97%). The main complications were transient, mild, postoperative hypernatremia; transient reduction or cessation of tear production (25 eyes in 18 dogs); permanent (five dogs) or prolonged (nine dogs) diabetes insipidus; and secondary hypothyroidism. Normal tear production had resumed in all but one case after a median period of 10 weeks. In the second case series (27 through 52), the hospitalization period was shorter, the number of dry eyes fewer, the survival fraction greater, and the postoperative mortality lower than in the first series. In 15 dogs in which remission was obtained, postoperative CT images suggested the presence of small pituitary remnants; in 1 of these, hyperadrenocorticism recurred. In 46 dogs, the histological diagnosis was pituitary adenoma. CONCLUSIONS: Microsurgical transsphenoidal hypophysectomy in dogs with PDH is an effective method of treatment in specialized veterinary institutions having access to advanced pituitary imaging techniques. Postoperative CT findings do not correlate well with remission or subsequent recurrence of hyperadrenocorticism. CLINICAL RELEVANCE: The neurosurgeon performing hypophysectomies must master a learning curve and must be familiar with the most frequent complications of the operation to recognize them as early as possible and to treat them immediately and effectively. Urinary C/C ratios are sensitive indicators for the assessment of remission and recurrence of hyperadrenocorticism.