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931.
An implicit tenet of modern search heuristics is that there is a mutually exclusive balance between two desirable goals: search
diversity (or distribution), i.e., search through a maximum number of distinct areas, and, search intensity, i.e., a maximum
search exploitation within each specific area. We claim that the hypothesis that these goals are mutually exclusive is false
in parallel systems. We argue that it is possible to devise methods that exhibit high search intensity and high search diversity
during the whole algorithmic execution. It is considered how distance metrics, i.e., functions for measuring diversity (given
by the minimum number of local search steps between two solutions) and coordination policies, i.e., mechanisms for directing
and redirecting search processes based on the information acquired by the distance metrics, can be used together to integrate
a framework for the development of advanced collective search methods that present such desiderata of search intensity and
search diversity under simultaneous coexistence. The presented model also avoids the undesirable occurrence of a problem we
refer to as the ‘ergometric bike phenomenon’. Finally, this work is one of the very few analysis accomplished on a level of
meta-meta-heuristics, because all arguments are independent of specific problems handled (such as scheduling, planning, etc.),
of specific solution methods (such as genetic algorithms, simulated annealing, tabu search, etc.) and of specific neighborhood
or genetic operators (2-opt, crossover, etc.). 相似文献
932.
This paper presents a general algorithmic framework for computing the IPA derivatives of sample performance functions defined
on networks of fluid queues. The underlying network-model consists of bi-layered hybrid dynamical systems with continuous-time
dynamics at the lower layer and discrete-event dynamics at the upper layer. The linearized system, computed from the sample
path via a discrete-event process, yields fairly simple algorithms for the IPA derivatives. As an application-example, the
paper discusses loss and workload performance functions in a tandem network with congestion control, subjected to signal delays. 相似文献
933.
Finding the optimal design for a discrete event dynamic system (DEDS) is in general difficult due to the large search space
and the simulation-based performance evaluation. Various heuristics have been developed to find good designs. An important
question is how to quantify the goodness of the heuristic designs. Inspired by the Ordinal Optimization, which has become
an important tool for optimizing DEDS, we provide a method which can quantify the goodness of the design. By comparing with
a set of designs that are uniformly sampled, we measure the ordinal performances of heuristic designs, i.e., we quantify the
ranks of all (or some of) the heuristic designs among all the designs in the entire search space. The mathematical tool we
use is the Hypothesis Testing, and the probability of making Type II error in the quantification is controlled to be under
a very low level. The method can be used both when the performances of the designs can be accurately evaluated and when such
performances are estimated by a crude but computationally easy model. The method can quantify both heuristics that output
a single design and that output a set of designs. The method is demonstrated through numerical examples. 相似文献
934.
Santosh Biswas Dipankar Sarkar Siddhartha Mukhopadhyay Amit Patra 《Discrete Event Dynamic Systems》2010,20(3):349-376
Failure diagnosability has been widely studied for discrete event system (DES) models because of modeling simplicity and computational
efficiency due to abstraction. In the literature it is often held that for diagnosability, such models can be used not only
for systems that fall naturally in the class of DES but also for the ones traditionally treated as continuous variable dynamic
systems. A class of algorithms for failure diagnosability of DES models has been successfully developed for systems where
fairness is not a part of the model. These algorithms are based on detecting cycles in the normal and the failure model that
look identical. However, there exist systems with all transitions fair where the diagnosability condition that hinges upon
this feature renders many failures non-diagnosable although they may actually be diagnosable by transitions out of a cycle.
Hence, the diagnosability conditions based on cycle detection need to be modified to hold for many real-world systems where
all transitions are fair. In this work, however, it is shown by means of an example that a system may have some transitions
fair and some unfair. A new failure diagnosability mechanism is proposed for DES models with both fair and unfair transitions.
Time complexity for deciding diagnosability of DES models with fair and unfair transitions is analyzed and compared with the
time complexities of other DES diagnosability analysis methods reported in the literature. 相似文献
935.
Benjamin Van Roy 《Discrete Event Dynamic Systems》2010,20(3):307-324
We study approaches that fit a linear combination of basis functions to the continuation value function of an optimal stopping
problem and then employ a greedy policy based on the resulting approximation. We argue that computing weights to maximize
expected payoff of the greedy policy or to minimize expected squared-error with respect to an invariant measure is intractable.
On the other hand, certain versions of approximate value iteration lead to policies competitive with those that would result
from optimizing the latter objective. 相似文献
936.
We consider Discrete Event Systems (DES) involving tasks with real-time constraints and seek to control processing times so
as to minimize a cost function subject to each task meeting its own constraint. It has been shown that the off-line version
of this problem can be efficiently solved by the Critical Task Decomposition Algorithm (CTDA) (Mao et al., IEEE Trans Mobile Comput 6(6):678–688, 2007). In the on-line version, random task characteristics (e.g., arrival times) are not known in advance. To bypass this difficulty,
worst-case analysis may be used. This, however, does not make use of probability distributions and results in an overly conservative
solution. In this paper, we develop a new approach which does not rely on worst-case analysis but provides a “best solution
in probability” efficiently obtained by estimating the probability distribution of sample-path-optimal solutions. We introduce
a condition termed “non-singularity” under which the best solution in probability leads to the on-line optimal control. Numerical
examples are included to illustrate our results and show substantial performance improvements over worst-case analysis. 相似文献
937.
938.
Improved PLSOM algorithm 总被引:1,自引:1,他引:0
Erik Berglund 《Applied Intelligence》2010,32(1):122-130
The original Parameter-Less Self-Organising Map (PLSOM) algorithm was introduced as a solution to the problems the Self-Organising
Map (SOM) encounters when dealing with certain types of mapping tasks. Unfortunately the PLSOM suffers from over-sensitivity
to outliers and over-reliance on the initial weight distribution. The PLSOM2 algorithm is introduced to address these problems
with the PLSOM. PLSOM2 is able to cope well with outliers without exhibiting the problems associated with the standard PLSOM
algorithm. The PLSOM2 requires very little computational overhead compared to the standard PLSOM, thanks to an efficient method
of approximating the diameter of the inputs, and does not rely on a priori knowledge of the training input space. This paper
provides a discussion of the reasoning behind the PLSOM2 and experimental results showing its effectiveness for mapping tasks. 相似文献
939.
Translating between dissimilar languages requires an account of the use of divergent word orders when expressing the same
semantic content. Reordering poses a serious problem for statistical machine translation systems and has generated a considerable
body of research aimed at meeting its challenges. Direct evaluation of reordering requires automatic metrics that explicitly
measure the quality of word order choices in translations. Current metrics, such as BLEU, only evaluate reordering indirectly.
We analyse the ability of current metrics to capture reordering performance. We then introduce permutation distance metrics
as a direct method for measuring word order similarity between translations and reference sentences. By correlating all metrics
with a novel method for eliciting human judgements of reordering quality, we show that current metrics are largely influenced
by lexical choice, and that they are not able to distinguish between different reordering scenarios. Also, we show that permutation
distance metrics correlate very well with human judgements, and are impervious to lexical differences. 相似文献
940.
Abhishek Arun Barry Haddow Philipp Koehn Adam Lopez Chris Dyer Phil Blunsom 《Machine Translation》2010,24(2):103-121
Recent advances in statistical machine translation have used approximate beam search for NP-complete inference within probabilistic
translation models. We present an alternative approach of sampling from the posterior distribution defined by a translation
model. We define a novel Gibbs sampler for sampling translations given a source sentence and show that it effectively explores
this posterior distribution. In doing so we overcome the limitations of heuristic beam search and obtain theoretically sound
solutions to inference problems such as finding the maximum probability translation and minimum risk training and decoding. 相似文献