Core–periphery structure

For the language theory, see Center versus periphery.

For the concept in world-systems theory, see Core countries and Periphery countries.

Network with an idealized core–periphery structure

Core–periphery structure is a network theory model.

Models of core–periphery structures

There are two main intuitions behind the definition of core–periphery network structures; one assumes that a network can only have one core, whereas the other allows for the possibility of multiple cores. These two intuitive conceptions serve as the basis for two modes of core–periphery structures.

Discrete model

This model assumes that there are two classes of nodes. The first consists of a cohesive core sub-graph in which the nodes are highly interconnected, and the second is made up of a peripheral set of nodes that is loosely connected to the core. In an ideal core–periphery matrix, core nodes are adjacent to other core nodes and to some peripheral nodes while peripheral nodes are not connected with other peripheral nodes (Borgatti & Everett, 2000, p. 378). This requires, however, that there be an a priori partition that indicates whether a node belongs to the core or periphery.

In this idealized matrix, the blue block represents core-core links, the white block represents the absence of periphery links and the red quadrants represent core–periphery ties.

Continuous model

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. 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

Hubs are commonly found in empirical networks and pose a problem for community detection as they usually have strong ties to many communities. Identifying core–periphery structures can help circumvent this problem by categorizing hubs as part of the network's core (Rombach et al., 2014, p. 160). Likewise, though all core nodes have high centrality measures, not all nodes with high centrality measures belong to the core. It is possible to find that a set of highly central nodes in a graph does not make an internally cohesive subgraph (Borgatti & Everett, 2000)...

Use in economics and geography

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 Thünen's Isolated State (1826). 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.

Building on this, the concept of core and periphery has been further developed in both the field of development economics and economic geography in an attempt to explain why certain areas become rich and industrialised, while other areas remain poor and resource-dependent ,. Prebisch's work emphasised the fact that the world trade patterns were intended to favour industrialised core countries, leaving the peripheral countries dependent on the export of raw materials and limited in their economic development . This idea resulted in the development of the dependency theory and Wallerstein's world-systems approach, which separates the world into core, semi-periphery and periphery regions in order to explain the historical and structural relationships that determine economic results .

Economic geographers have extended these ideas by demonstrating that population density, transport costs, and increasing returns can lead to the natural development of core regions with dense networks of industry and urbanisation and to backward regions flying by on the wings of John Kenneth Galbraith's envisionable future. Economic geographers have extended these ideas by demonstrating how population density, transport costs, and increasing returns will naturally produce core and lagging regions flying by on the wings of John Kenneth Galbraith's envisionable future. Paul Krugman's New Economic Geography describes the tendency for goods and labour firms to converge to places in economically pleasing areas, producing a self-reinforcing core. This accounts for the emergence of regional economic centres, metropolitan hierarchies and global value chains. Peripheral regions, meanwhile, tend to specialise in providing the raw materials or cheaper labour for the core.

Modern-day applications of the core-periphery calculus are in the analysis of regional inequalities, urban development, and uneven distribution of wealth and infrastructure in countries and on the global level. For example, large cities or industrial areas commonly play a predominant role in the national economy, and rural areas or less developed regions are left to rely on them. This framework is used by policymakers in designing strategies for regional development, infrastructure investment and trade planning .

Overall, the core-periphery model is a powerful tool in understanding spatial inequalities in the economy. By considering the connection between trade patterns in the past, industrial concentration, and urban hierarchies, it demonstrates the role of economic processes in channelling the distribution of wealth, population, and opportunity between regions.

See also

• World-systems theory
• Core countries
• Semi-periphery countries
• Periphery countries
• Degeneracy (graph theory)

References

• Borgatti, S. P., & Everett, M. G. (1999). Models of core /periphery structures. Social Networks, 21, 375–395. doi:10.1016/S0378-8733(99)00019-2