82,894 research outputs found
Efficient prime counting and the Chebyshev primes
The function \epsilon(x)=\mbox{li}(x)-\pi(x) is known to be positive up to
the (very large) Skewes' number. Besides, according to Robin's work, the
functions \epsilon_{\theta}(x)=\mbox{li}[\theta(x)]-\pi(x) and
\epsilon_{\psi}(x)=\mbox{li}[\psi(x)]-\pi(x) are positive if and only if
Riemann hypothesis (RH) holds (the first and the second Chebyshev function are
and ,
respectively, \mbox{li}(x) is the logarithmic integral, and
are the M\"obius and the Von Mangoldt functions). Negative jumps
in the above functions , and
may potentially occur only at (the set of primes). One
denotes j_p=\mbox{li}(p)-\mbox{li}(p-1) and one investigates the jumps ,
and . In particular, , and
for . Besides, for any odd p \in
\mathcal{\mbox{Ch}}, an infinite set of so-called {\it Chebyshev primes } with
partial list . We establish a few properties of the set
\mathcal{\mbox{Ch}}, give accurate approximations of the jump
and relate the derivation of \mbox{Ch} to the explicit Mangoldt formula for
. In the context of RH, we introduce the so-called {\it Riemann
primes} as champions of the function (or of the function
). Finally, we find a {\it good} prime counting function
S_N(x)=\sum_{n=1}^N \frac{\mu(n)}{n}\mbox{li}[\psi(x)^{1/n}], that is found
to be much better than the standard Riemann prime counting function.Comment: 15 pages section 2.2 added, new sequences added, Fig. 2 and 3 are ne
The Synthesis and Characterization of New, Robust Titanium (IV) Scorpionate Complexes
Titanium complexes possessing sterically encumbered ligands have allowed for the preparation of reactive moieties (imido, alkylidene and alkylidyne species) relevant to reactions such as olefin polymerization and alkyne hydroamination. For this reason, we have targeted robust scorpionate ancillary ligands to support reactive titanium centers. Thus, a series of titanium complexes were synthesized using an achiral oxazoline-based scorpionate ligand, tris(4,4-dimethyl-2-oxazolinyl)phenyl borate [To^M^]^-^ as well as the related chiral ligand, tris(4-isopropyl-2-oxazolinyl)phenyl borate [To^P^]^-^. The complex [Ti(κ^3^- To^M^)Cl~3~] was prepared in moderate yield (43%) by the rapid (<1 min at room temperature) reaction of Li[To^M^] and TiCl~4~ in methylene chloride; this new compound was characterized by ^1^H NMR spectroscopy as the expected C~3v~-symmetric species. One route to Ti (IV) alkyls involves salt metathesis; accordingly, syntheses of [To^M^]Ti alkyl complexes by interaction of [Ti(κ^3^-To^M^)Cl~3~] and one or three equivalents of alkylating agents, such as benzyl potassium (KCH~2~C~6~H~5~), trimethylsilylmethyl
lithium (LiCH~2~Si(CH~3~) ~3~), or neopentyl lithium (LiCH~2~C(CH~3~)~3~) are currently under investigation. The complexes [Ti(=NBut) (κ~3~-To^M^)(Cl)(Bu^t^py)] (Bu^t^py=4 tert-butylpyridine) and [Ti(=NBu^t^) (κ~3~-To^P^)(Cl)(Bu^t^py)] were synthesized by reaction of the known Ti imido [Ti(=NBu^t^)(Cl)~2~(Bu^t^py)~2~] with Li[To^M^] or Li[To^P^], respectively, by stirring overnight in methylene chloride at ambient temperature. The complexes were identified using ^1^H NMR spectroscopy, ^1^H-^13^C HMQC and ^1^H-^15^N HMBC correlation experiments
Ferrocenyl hydroxymethylphosphines (ηâ”-Câ Hâ )Fe[ηâ”â»Câ HâP(CHâOH)â] and 1,1âČ-[Fe{ηâ”-Câ HâP(CHâOH)â}â] and their chalcogenide derivatives
The ferrocenyl hydroxymethylphosphines FcP(CHâOH)â [Fc=(ηâ”-Câ
Hâ
)Fe(ηâ”-Câ
Hâ)] and 1,1âČ-FcâČ[P(CHâOH)â]â [FcâČ=Fe(ηâ”â»Câ
Hâ)â] were prepared by reactions of the corresponding primary phosphines FcPHâ and 1,1âČ-FcâČ(PHâ)â with excess aqueous formaldehyde. The crystal structure of FcP(CHâOH)â was determined and compared with the known ferrocenyl hydroxymethylphosphine FcCHâP(CHâOH)â. The chalcogenide derivatives FcP(E)(CHâOH)â and 1,1âČ-FcâČ[P(E)(CHâOH)â]â (E=O, S, Se) were prepared and fully characterised. Crystal structure determinations on FcP(O)(CHâOH)â and FcP(S)(CHâOH)â were performed, and the hydrogen-bonding patterns are compared with related compounds. The sulfide shows no hydrogen-bonding involving the phosphine sulfide group, in contrast to other reported ferrocenyl hydroxymethylphosphine sulfides. The platinum complex cis-[PtClâ{FcP(CHâOH)â}â] was prepared by reaction of 2 mol equivalents of FcP(CHâOH)â with [PtClâ(1,5-cyclo-octadiene)], and was characterised by 31P-NMR spectroscopy and negative ion electrospray mass spectrometry, which gave a strong [M+Cl]â» ion
Feshbach resonances in ultracold ^{6,7}Li + ^{23}Na atomic mixtures
We report a theoretical study of Feshbach resonances in Li + Na
and Li + Na mixtures at ultracold temperatures using new accurate
interaction potentials in a full quantum coupled-channel calculation. Feshbach
resonances for in the initial collisional open channel LiNa are found to agree with previous
measurements, leading to precise values of the singlet and triplet scattering
lengths for the LiNa pairs. We also predict additional Feshbach
resonances within experimentally attainable magnetic fields for other collision
channels.Comment: 4 pages, 3 figure
Feshbach resonances in mixtures of ultracold Li and Rb gases
We report on the observation of two Feshbach resonances in collisions between
ultracold Li and Rb atoms in their respective hyperfine ground
states and . The resonances show up as trap losses
for the Li cloud induced by inelastic Li-Rb-Rb three-body collisions. The
magnetic field values where they occur represent important benchmarks for an
accurate determination of the interspecies interaction potentials. A broad
Feshbach resonance located at 1066.92 G opens interesting prospects for the
creation of ultracold heteronuclear molecules. We furthermore observe a strong
enhancement of the narrow p-wave Feshbach resonance in collisions of Li
atoms at 158.55 G in the presence of a dense Rb cloud. The effect of the
Rb cloud is to introduce Li-Li-Rb three-body collisions occurring at a
higher rate than Li-Li-Li collisions.Comment: 4 pages, 3 figure
Analytical Approximations for the Collapse of an Empty Spherical Bubble
The Rayleigh equation 3/2 R'+RR"+p/rho=0 with initial conditions R(0)=Rmax,
R'(0)=0 models the collapse of an empty spherical bubble of radius R(T) in an
ideal, infinite liquid with far-field pressure p and density rho. The solution
for r=R/Rmax as a function of time t=T/Tcollapse, where R(Tcollapse)=0, is
independent of Rmax, p, and rho. While no closed-form expression for r(t) is
known we find that s(t)=(1-t^2)^(2/5) approximates r(t) with an error below 1%.
A systematic development in orders of t^2 further yields the
0.001%-approximation r*(t)=s(t)[1-a Li(2.21,t^2)], where a=-0.01832099 is a
constant and Li is the polylogarithm. The usefulness of these approximations is
demonstrated by comparison to high-precision cavitation data obtained in
microgravity.Comment: 5 pages, 2 figure
An Extremely Lithium-Rich Bright Red Giant in the Globular Cluster M3
We have serendipitously discovered an extremely lithium-rich star on the red
giant branch of the globular cluster M3 (NGC 5272). An echelle spectrum
obtained with the Keck I HIRES reveals a Li I 6707 Angstrom resonance doublet
of 520 milli-Angstrom equivalent width, and our analysis places the star among
the most Li-rich giants known: log[epsilon(Li)] ~= +3.0. We determine the
elemental abundances of this star, IV-101, and three other cluster members of
similar luminosity and color, and conclude that IV-101 has abundance ratios
typical of giants in M3 and M13 that have undergone significant mixing. We
discuss mechanisms by which a low-mass star may be so enriched in Li, focusing
on the mixing of material processed by the hydrogen-burning shell just below
the convective envelope. While such enrichment could conceivably only happen
rarely, it may in fact regularly occur during giant-branch evolution but be
rarely detected because of rapid subsequent Li depletion.Comment: 7-page LaTeX file, including 2 encapsulated ps figures + 1 table;
accepted for publication in the Astrophysical Journal Letter
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