Universal finite group extensions and a non-splitting theorem

LetG andK be finite groups whose orders have a common prime divisor. Then there is a groupK ^* closely related toK for which there is a non-split extension ofK ^* byG. I wish to express thanks to the Mathematics Institute of the Hebrew University of Jerusalem for its hospitality from September to December 1972, and to Dr. Avinoam Mann for his helpful comments.     

A comparison of classical and quantum-mechanical ‘ring-current’ theories

The proton resonance spectra of a series of 2,7-disubstituted-4,5-bis(2-pyridyl)phenanthrene-3,6-diols are examined with emphasis on the shielding effects produced by aromatic ‘ring currents.’ In this series, two pyridine rings are rigidly held less than 3.4 Å apart and with considerable overlap. The ‘ringcurrent’ shieldings found for above-the-plane positions are in good agreement with the Johnson-Bovey theory, whereas a recently proposed quantum-mechanical theory underestimates the shieldings by a factor of nearly three.     

On the absence of an s-band energy gap in niobium

No evidence for a superconducting tunneling s-band energy gap in single crystal Nb has been observed for any of the main symmetry directions investigated in Nb-oxide-Pb_0.7Bi_0.3 and Nb-oxide-In junctions.     

On the use of photoelastic composite models

Emphasis in the paper is placed on establishing the validity of employing photoelastic composite models to study assumptions used in composite-material analysis. Two approaches for the derivation of material constants for plane-stress layered composite materials are presented. Basic assumptions of the theories are studied. The theoretical and experimental equivalent moduli for test specimens loaded in tension and in shear are also compared.     

Ion photodesorption from argon multilayers

Received: 16 June 1997/Accepted: 26 October 1997     

Ba(SCN)2(H2O) complex of the cis-syn-cis isomer of 2,5,8,15,18,21-hexaoxatricyclo[20,4,0,09,14]hexacosane(dicyclohexyl-18-crown-6) [Ba(SCN)2(H2O)(C20H36O6)]

The structure of the title compound has been redetermined. Hydrogenation of dibenzo-18-crown-6 yields two conformational isomers which were proposed to be the cis-syn-cis and the cis-trans-cis isomers of dicyclohexyl-18-crown-6. The previous study had established that the title complex contains the ligand which has the cis-syn-cis conformation but had resulted in some inaccurate bond lengths and also in an uncertainty regarding the presence of a water of hydration in the crystal. This study confirmed the report of the general conformation of the complex, resulted in more precise bond lengths and clearly established that a water of hydration was present at the crystal.