The stable nature of wealth-to-GDP was first recognised by the Swedish economist Knut Wicksell, and he used it in much of his work in the late 19th century. In mathematical terms, the stable nature of the capital-to-output ratio was first expressed by Paul Douglas (economist) and Charles Cobb (mathematician). Their work in the 1920s turned into what is now known as the Cobb-Douglas production function. The standard version of that production function with only two input factors – capital (K) and labour (L) – states that output (Y) can be defined as follows: Y = AKαL1-α
