收费全文 | 4792篇 |
免费 | 236篇 |
国内免费 | 5篇 |
2024年 | 9篇 |
2023年 | 61篇 |
2022年 | 99篇 |
2021年 | 130篇 |
2020年 | 173篇 |
2019年 | 198篇 |
2018年 | 285篇 |
2017年 | 305篇 |
2016年 | 302篇 |
2015年 | 164篇 |
2014年 | 192篇 |
2013年 | 544篇 |
2012年 | 254篇 |
2011年 | 264篇 |
2010年 | 189篇 |
2009年 | 170篇 |
2008年 | 242篇 |
2007年 | 218篇 |
2006年 | 218篇 |
2005年 | 149篇 |
2004年 | 125篇 |
2003年 | 115篇 |
2002年 | 69篇 |
2001年 | 50篇 |
2000年 | 38篇 |
1999年 | 38篇 |
1998年 | 27篇 |
1997年 | 22篇 |
1996年 | 21篇 |
1995年 | 28篇 |
1994年 | 20篇 |
1993年 | 28篇 |
1992年 | 21篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 12篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 15篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 19篇 |
1980年 | 11篇 |
1979年 | 14篇 |
1977年 | 9篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1970年 | 7篇 |
Gauss’s quadratic reciprocity theorem is among the most important results in the history of number theory. It’s also among the most mysterious: since its discovery in the late eighteenth century, mathematicians have regarded reciprocity as a deeply surprising fact in need of explanation. Intriguingly, though, there’s little agreement on how the theorem is best explained. Two quite different kinds of proof are most often praised as explanatory: an elementary argument that gives the theorem an intuitive geometric interpretation, due to Gauss and Eisenstein, and a sophisticated proof using algebraic number theory, due to Hilbert. Philosophers have yet to look carefully at such explanatory disagreements in mathematics. I do so here. According to the view I defend, there are two important explanatory virtues—depth and transparency—which different proofs (and other potential explanations) possess to different degrees. Although not mutually exclusive in principle, the packages of features associated with the two stand in some tension with one another, so that very deep explanations are rarely transparent, and vice versa. After developing the theory of depth and transparency and applying it to the case of quadratic reciprocity, I draw some morals about the nature of mathematical explanation.
相似文献Prenatal smoke exposure (PSE) is a risk factor for adverse outcomes in the offspring, including those affecting psychological development. However, it is uncertain whether these associations are the direct result of PSE or other confounding factors. The aim of this study was to examine the possible relationship between PSE and behavioral development in children at 7.5 years of age, considering several prenatal, neonatal and postnatal covariates. A cohort of 266 mother-child pairs was followed from the first trimester of pregnancy until the children reached 7.5 years of age. PSE was assessed using a questionnaire from prenatal clinical records and corroborated by plasma cotinine determinations in the first and second trimesters and in the cord. Mother-child pairs were classified into one of four groups: unexposed, exposed to passive smoking, first trimester active smoking only and active smoking throughout pregnancy. Child behavior was assessed using the Child Behavior Checklist for ages 6–18 and the Childhood Autism Spectrum Test. In multiple linear regression models, smoking during pregnancy was associated with higher scores in affective problems (β?=?0.298; p?=?0.004). No significant associations were found between smoking during pregnancy and externalizing problems. Findings indicate that PSE is negatively associated with behavioral development in childhood.
相似文献