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RT 在读Romer（1993）的rational asset-price movement 论文 其中第二个模型中提到： theseassumption capture the fact that trading on the basis of information is more costly when trading is done immediately.If, for example, there are transactions costs, trading immediately requires incurring those coses. But if trading is postponed, other considerations, such as liquidity needs or the desire to trade  on the basis of other information,may cause the investor to trade at some later date;at this point there are no costs associated with incorporating the information into demand. 文中提到的立即trade就有cost，延迟交易就没，我不太了解怎么产生的？请帮忙解释一下~~~~ 谢谢各位！！！