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Programmatically Interpretable Reinforcement Learning (PIRL) encodes policies in human-readable computer programs. Novel algorithms were recently introduced with the goal of handling the lack of gradient signal to guide the search in the space of programmatic policies. Most of such PIRL algorithms first train a neural policy that is used as an oracle to guide the search in the programmatic space. In this paper, we show that such PIRL-specific algorithms are not needed, depending on the language used to encode the programmatic policies. This is because one can use actor-critic algorithms to directly obtain a programmatic policy. We use a connection between ReLU neural networks and oblique decision trees to translate the policy learned with actor-critic algorithms into programmatic policies. This translation from ReLU networks allows us to synthesize policies encoded in programs with if-then-else structures, linear transformations of the input values, and PID operations. Empirical results on several control problems show that this translation approach is capable of learning short and effective policies. Moreover, the translated policies are at least competitive and often far superior to the policies PIRL algorithms synthesize.

In the present work we report on the current progress of the recently constructed GW attosecond extreme ultraviolet (XUV) source developed at the Institute of Electronic Structure and Laser of the Foundation for Research and Technology-Hellas (I.E.S.L-FO.R.T.H.). By the implementation of a compact-collinear polarization gating arrangement, the generation of a broadband, coherent XUV quasi-continuum produced by the interaction of a many-cycle infrared field with a gas phase medium is achieved. The spectral width of the XUV emission generated in Xenon, is spanning in the range of 17-32 eV and can support isolated pulses of duration in the range from 0.4 fs to 1.3 fs and pulse energy in the 1 {\mu}J level.Theoretical calculations, taking into account the experimental conditions of this work, are supporting the observations, offering also an insight regarding the temporal profile of the emitted radiation. Finally, the high intensity of the produced XUV pulses has been confirmed by investigating the two-XUV-photon double ionization process of Argon atoms. The demonstrated results inaugurate the capability of the beamline to produce intense isolated attosecond pulses towards their exploitation in studies of non-linear XUV processes, attosecond pulse metrology and XUV pump XUV probe experiments., Comment: 18 pages, 6 figures

Two XUV-photon double ionization of Ne, induced by an intense few-pulse attosecond train with a ~ 4 fs envelope duration is investigated experimentally and theoretically. The experiment is performed at ELI-ALPS utilizing the recently constructed 10 Hz gas phase high-order harmonic generation SYLOS GHHG-COMPACT beamline. A total pulse energy up to ~1 {\mu}J generated in Argon in conjunction with high reflectivity optics in the XUV region, allowed the observation of the doubly charged state of Ne induced by 40 eV central XUV photon energies. The interaction of the intense attosecond pulse train with Ne is also theoretically studied via a second-order time dependent perturbation theory equations-of-motion. The results of this work, combined with the feasibility of conducting XUV-pump-XUV-probe experiments, constitute a powerful tool for many potential applications. Those include attosecond pulse metrology as well as time resolved investigations of the dynamics underlying direct and sequential double ionization and their electron correlation effects.

In this work, we demonstrate experimentally the efficient generation and tunability of energetic highly-elliptical high-harmonics in Ar gas, driven by intense two-color counter rotating laser electric fields. A bi-chromatic beam tailored by a MAZEL-TOV apparatus generates HHG, where the output spectrum of the highly elliptical HHG radiation can be tuned for a energy range of \DeltaE\HF150meV in the spectral range of ~20 eV with energy per pulse EXUV~400 nJ at the source. Furthermore we employ time-dependent density functional simulations to probe in-depth the dependence of the harmonic ellipticity and the strength of the isolated atto pulses on the driving field parameters and demonstrate the robustness of the HHG with the bichromatic field. We show how by properly tuning the central frequency of the second harmonic, the central frequency of the extreme ultraviolet (XUV) high harmonic radiation is continuously tuned. The demonstrated energy values largely exceed the output energy from many other laser driven attosecond sources reported so far and prove to be sufficient for inducing (along with tight XUV focusing geometries) the nonlinear processes in the atomic system. We envisage that such tunable energetic highly-elliptical HHG spectra can remove the facility restrictions from requirements of few-cycle driving pulses for isolated circular attosecond pulse generation., Comment: arXiv admin note: text overlap with arXiv:2106.08690

In this study the generation of energetic coherent extreme ultraviolet (XUV) radiation with controlled polarization, is reported. The XUV radiation results from the process of high harmonic generation (HHG), in a gas phase atomic medium, driven by an intense two-color circularly polarized counter-rotating laser field, under loose focusing geometry conditions. The energy of the XUV radiation emitted per laser pulse is found to be of the order of ~100 nJ with the spectrum spanning from 17 to 26 eV. The demonstrated energy values along with tight focusing geometries is sufficient to induce nonlinear processes and challenges the perspectives for ultrafast investigations of chiral phenomena in the XUV spectral region., Comment: 17 pages, 5 Figures

We report the observation of multiple ionization of Argon through multi-XUV-photon absorption induced by an unprecedentedly powerful laser driven high-harmonic-generation source. Comparing the measured intensity dependence of the yield of the different Argon charge states with numerical calculations we can infer the different channels -direct and sequential- underlying the interaction. While such studies were feasible so far only with FEL sources, this work connects highly-non-linear-XUV-processes with the ultra-short time scales, inherent to the harmonic pulses, and highlights the advanced perspectives of emerging large scale laser research infrastructures., Comment: 19 pages, 6 figures

The field of neuroimaging has truly become data rich, and novel analytical methods capable of gleaning meaningful information from large stores of imaging data are in high demand. Those methods that might also be applicable on the level of individual subjects, and thus potentially useful clinically, are of special interest. In the present study, we introduce just such a method, called nonlinear functional mapping (NFM), and demonstrate its application in the analysis of resting state fMRI from a 242-subject subset of the IMAGEN project, a European study of adolescents that includes longitudinal phenotypic, behavioral, genetic, and neuroimaging data. NFM employs a computational technique inspired by biological evolution to discover and mathematically characterize interactions among ROI (regions of interest), without making linear or univariate assumptions. We show that statistics of the resulting interaction relationships comport with recent independent work, constituting a preliminary cross-validation. Furthermore, nonlinear terms are ubiquitous in the models generated by NFM, suggesting that some of the interactions characterized here are not discoverable by standard linear methods of analysis. We discuss one such nonlinear interaction in the context of a direct comparison with a procedure involving pairwise correlation, designed to be an analogous linear version of functional mapping. We find another such interaction that suggests a novel distinction in brain function between drinking and non-drinking adolescents: a tighter coupling of ROI associated with emotion, reward, and interoceptive processes such as thirst, among drinkers. Finally, we outline many improvements and extensions of the methodology to reduce computational expense, complement other analytical tools like graph-theoretic analysis, and allow for voxel level NFM to eliminate the necessity of ROI selection., Comment: 21 pages, 12 figures, and 1 table

We prove that the crossed product AxG of a unital finitely generated MF algebra A by a discrete finitely generated amenable residually finite group G is an MF algebra, provided that the action is almost periodic. This generalizes a result of Hadwin and Shen. We also construct two examples of crossed product C*-algebras whose BDF Ext semigroups are not groups., Comment: 9 pages

We define a broad class of crossed product C*-algebras of the form C(G)xG, where G is a discrete countable amenable residually finite group, and G is a profinite completion of G. We show that they are unital separable simple nuclear quasidiagonal C*-algebras, or real rank zero, stable rank one, with comparability of projections and with a unique trace.

We prove that the crossed product A x G of a separable, unital, quasidiagonal C*- algebra A by a discrete, countable, amenable, maximally almost periodic group G is quasidiagonal, provided that the action is almost periodic., Comment: Minor improvements