The history of hydrogen — the element that fills the world as we know it — consists of a most dramatic set of events. Hydrogen and helium atoms emerged a measly 379,000 years after the Big Bang. As the hot, dense plasma of protons, electrons and photons that was the universe began to cool and expand, electrons and protons gathered to form atoms. Four hundred million years later stars — such as ...

Our aim is for intelligent tutoring agents to replace traditional and even online textbooks with personalized, adaptive, one-to-one instruction. We focus on science subjects, and describe an approach to answering hypothetical questions from the student, such as “Would cellular respiration continue in the absence of oxygen?”

T he Task Force on Community Preventive Services presents recommendations in this supplement to the American Journal of Preventive Medicine relating to the transfer of juveniles to the adult criminal justice system. 1,2 The questions the Task Force considered are whether transfers reduce or prevent violent crimes by people younger than 18 by means either of individual deterrence (reducing futur...

He had an additional reason to be inconsolable. What 5 year old would be happy without anything to eat and nothing on the horizon to correct this deficit? Not even the nurse could solve this problem. Armed with two drinks, the oral premedication and apple juice to wash it down, she tried to help gain some control by offering what had been withheld to this point. This smart little boy, however, ...

Our aim is for intelligent tutoring agents to replace traditional and even online textbooks with personalized, adaptive, one-to-one instruction. We focus on science subjects, and describe an approach to answering hypothetical questions from the student, such as “Would cellular respiration continue in the absence of oxygen?”

Claim 1. In any equilibrium, we have  1  0. Proof. First, we show that in any equilibrium we must have ∗  0. Suppose to the contrary that we had ∗ = 0. Then by posting a price 0 ∈ ( ̄) in period 1, an inactive firm would be sure to sell, making the deviation profitable, a contradiction to equilibrium. Hence we must have ∗  0 . Next, suppose to the contrary to the statement of Claim 1 ...