This model allows for the existence of three or more partitions of node classes. However, including more classes makes modifications to the discrete model more difficult.{{needs clarificationclarify|date=August 2019}} Borgatti & Everett (1999) suggest that, in order to overcome this problem, each node be assigned a measure of ‘coreness’ that will determine its class. Nevertheless, the threshold of what constitutes a high ‘coreness’ value must be justified theoretically.
==Discussion==
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==Uses in economics==
The concept was first introduced into economics as "centre-periphery" by [[Raúl Prebisch]] in the 1950s, but the origin of the idea could ultimately be traced back to [[Johann Heinrich von Thünen|Thünen]]'s ''Isolated State'' (1826).<ref>{{Cite journal |last1=Rama |first1=J. |last2=Hall |first2=J. |year=2021 |title=Raúl Prebisch and the evolving uses of ‘centre'centre-periphery’inperiphery'in economic analysis |journal=Review of Evolutionary Political Economy |volume=2 |issue=2 |pages=315-332315–332}}</ref> However, the qualitative notion that [[social networks]] can have a core–periphery structure has a long history in disciplines such as [[sociology]], [[international relations]] (Nemeth & Smith, 1985), and [[economics]] (Snyder & Kick, 1979). Observed trade flows and diplomatic ties among countries fit this structure. [[Paul Krugman]] (1991) suggests that when transportation costs are low enough manufacturers concentrate in a single region known as the core and other regions (the periphery) limit themselves to the supply of agricultural goods.
