# Topology of the World Tourism Web
## 2 Materials and Methods
### Basic SNA Elements Applied to Tourism  
The basic elements in SNA are nodes and the links between the nodes. Based on the setting, the nodes (or vertices) may represent persons/tourists, stakeholders, countries, or agents/actors. The links (or edges/arcs) are relations between the nodes, also referred to as ties. [PERSON] and [PERSON] [41] systematized possible ties. In tourism, the relations or ties may denote, for example, transactions (payment for goods and services, cultural exchange, etc. ), movements (physical movements of tourists from the country of origin to the country of destination, movement between locations in the destination country, virtual movements over websites, etc. ), or formal roles (relationship of accommodation units to agencies and/or to the government). Given the properties of the relationship represented by links, a link may be undirected or directed. For example, if the movement of a tourist from the country of origin to the destination country is observed, a network representing that phenomenon should be a directed network. If one observes a cultural exchange between tourists, such phenomenon would be represented by an undirected network. The main difference (besides the analytical perspective) between directed and undirected links is based on how the links can be severed. For example, in the case of cultural exchange, where the link is undirected, it is assumed that both sides should agree to sever the link. In the case of tourists movements, a ban on traveling from the country of origin can be made without the consent of the destination country. Recently, it is estimated that international travel bans affected over 90% of the world population, according to [42]. For analytical purposes, those ties may be coded as binary, in which case they reveal whether a relationship between the two nodes exists or not, or they can be weighted, in which case a value is assigned to the link between the two nodes, enabling the examination of the network properties, given some quality (measured by assigned values). A path of the network denotes a sequence of links, where each node appears only once, while a walk is a sequence of links without the restriction of node distinction. In a directed graph, one must follow the direction of the link when determining either a walk or a path. A network is connected if every two nodes may be reached by some path. Networks and their components may create specific forms (forest, tree, star, cycle). For example, a tourism network based on a formal relationship between the guests, agencies, and the government would resemble a tree or even a forest; the recommendation network of one tourist might resemble a star; an efficient tour over the tourist attractions in a destination might resemble a cycle. Other networks can create uncharacteristic shapes, but some of their components or partitions may resemble specific forms. If the nodes can be systematized by a certain properties into subgroups, for example, in previously mentioned tourist movements, one might want to organize the countries of origin and destination countries according to the region or belonging continent, which can be done using coloring, allowing for an additional examination. So, networks can be observed as an overall connection of the nodes, but sometimes, for research purposes, it is interesting to observe network components or even each node. This is usually done by analysis, computing summary statistics that enable network characteristics' interpretation and comparison to similar networks. In addition, from the perspective of a network, network density, degree distributions, diameter, and average path length are usually calculated. Next, most often examined are network compounds, cliquishness, cohesiveness, and clustering [7,8,10]. In the case of tourism examinations, homophily between the countries of the same region may be examined, as well as the transitivity [6, 7, 8].