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Hayek's principal investigations in economics concerned capital, money, and the business cycle. Mises had earlier applied the concept of marginal utility to the value of money in his Theory of Money and Credit (1912), in which he also proposed an explanation for "industrial fluctuations" based on the ideas of the old British Currency School and of Swedish economist Knut Wicksell. Hayek used this body of work as a starting point for his own interpretation of the business cycle, elaborating what later became known as the "Austrian Theory of the Business Cycle". Hayek spelled out the Austrian approach in more detail in his book, published in 1929, an English translation of which appeared in 1933 as Monetary Theory and the Trade Cycle. There he argued for a monetary approach to the origins of the cycle. In his Prices and Production (1931), Hayek argued that the business cycle resulted from the central bank's inflationary credit expansion and its transmission over time, leading to a capital misallocation caused by the artificially low interest rates. Hayek claimed that "the past instability of the market economy is the consequence of the exclusion of the most important regulator of the market mechanism, money, from itself being regulated by the market process".

Hayek focused most of his economic works on the business cycle, money and what else?
capital