Patterns and Current Trends in African Migration to Europe

The history of African migration is tied to the period of the transatlantic slave trade often referred to as the “great migration.” Today, out of the estimated 250 million migrants in the world today, only 14% (36 million) are Africans with 26% speciﬁcally on the European continent. Although North Africans are Africa’s migration giant to Europe, the recent trend shows that migration of West Africans to Europe is also on the increase. African migration is propelled majorly by economic crisis, individual ambitions, political and armed violence. Irregular migration to Europe reached its peak in 2015 with about 1.8 million migrants crossing the Mediterranean Sea. However, this number has signiﬁcantly declined to fewer than 200,000 due to stricter border laws by the EU. Report shows that 50% of missing/dead migrants recorded on the Mediterranean Sea are of African origin. Irregular migrants are mostly men in their late 20’s with little education. Globalization plays a pivotal role in modern migration trends by being a major driving force. Although African migration may constitute signiﬁcant problems to Europe, there is ample evidence that the receiving countries in Europe remain the top gainers through migrants’ economic contributions.


Introduction
Both historical and modern antecedents suggest that international migration is an inevitable and complex phenomenon that touches political, economic and social lives of the interconnected world (Barriga, 2013;IOM, 2018). Prehistoric times addressed by the metaphor of Africa as the Cradle of Humanity need not even be mentioned here. The not-so-distant history of African migration to the world is tied to the occurrence of transatlantic slave trade in the colonial area which is often called the "great migration" (Curtin, 1997). During this era, colonialists took control of African human mobility, transported and channeled them to the New World and other regions to provide hard labor for the vitalization of American and European economies (Rodney, 1972). African migrants were forced to work on farmlands to produce cash crops such as tobacco, sugar, and cotton. In the late eighteenth century, which marked the peak of the slave trade era, about 80,000 Africans were annually mobilized to the Americas, primarily to Brazil and the Caribbean for intensive labor (Castles, de Haas, & Miller, 2013). It was also noted that almost 15 million slaves were transported to the same region between the sixteenth and the nineteenth century (Castles et al., 2013).
During the World Wars, a significant number of African soldiers and workers were recruited by the colonial imperials to support the military in Europe (Mafukidze, 2006;Rodney, 1972). Six years after World War I, around 100,000 Algerians were already working and living in France. About 250,000 North Africans mostly from Algeria and partly from Morocco and Tunisia migrated to France in the 1950s to supplement the postwar labor scarcity (Malka, 2018). Although Africans have liberated themselves through altercation of the nationalists with the colonial powers in the 1950s and 1970s, they are nevertheless tied to the colonial mentality that Europe and America remain the most economically viable regions to work, prosper and achieve their dreams. The African migration pattern of today is owned to colonization and post-colonization links with past colonial powers, and considered to shape the future trends of migration of Africans to Europe and the Americas (Adepoju, 2011;Appleyard, 1995).
Today, all continents have their own share (although at differing levels) in the sending and receiving of the world's migrant population. While sending countries are worried about brain drain and loosing of young working population to international migration, receiving countries are bothered by the socioeconomic consequences of hosting migrants' influx (Gheasi & Nijkamp, 2017). At present, Europe is the principal target of migrants from Africa and the Middle East, and faced with serious burdens at its borders (Danaj, Lazányi, & Bilan, 2018;Dokos, 2017;Flahaux & Hass, 2016) since the beginning of the political and economic crises in the two regions (Kugiel, 2016). On the other hand, migration remains central to Africa's socioeconomic landscape as it enhances prosperity, means of livelihood and acquirement of wealth for individuals and their households (Horwood, Forin, & Frouws, 2018). Recent statistics of the United Nations Department of Economic and Social Affairs show that 26% (nine million) of African migrants have moved to Europe (UNDESA, 2017), constituting 12% of all international migrants on European soil, according to the Food and Agriculture Organization of the United Nations (FAO, 2017).
Although movement of migrants from the Middle East to Europe seems to have leveled off in very recent times, the migration of people from Africa appears to be at its starting point (Malka, 2018). Four reasons are suggested for this trend (Malka, 2018). First, the African population is experiencing a dramatic expansion with a projected population of at least 2.5 billion by 2050 doubling the present estimate of 1.4 billion people (Worldometers, 2019). Second, given the projected huge population, there will be an unprecedented burden on economic resources for provision of employment, infrastructure and basic amenities in terms of electricity, health care, education and housing, most prominently in urban areas. Third, the present median age of Africans is 19.4 with almost 60% of people under age 25 (Worldometers, 2019). This age structure predisposes increased mobility, adaptability to new technologies and seeking of international opportunities. Fourth, the continent's compounding problems characterized by poverty, corruption, inadequate basic amenities for survival, political conflict, degradation of the environment and unemployment will persistently drive Africans to search for a better life abroad. However, it is important to note that this trend does not portend a mass exodus of Africans to Europe as majority of migration takes place within the African states (European Commission, 2018a).

African Migration Pattern: The Current Trends
African migration is both forced and voluntary, internal and transcontinental. Although migration out of Africa has been increasing in absolute terms over the last decades, the population of African migrants compared to the world's total migrants seems to be the lowest (African Union, 2017a). Out of the 258 million migrants in the world, 36 million (14%) were born in Africa. Statistics show that 53% of African migrants live in Africa, 26% in Europe, 12% in Asia and 7% in North America (UNDESA, 2017). These percentages almost remained steady for years 2000 and 2017 except for the 7% increase in the number of migrants within Africa (see Fig. 1). Current data also indicate that Europe is the common destination of African migrants after Africa herself (Flahaux & de Haas, 2016).
Economic crisis, individual ambitions, political and armed violence rocking many parts of Africa have substantially forced and motivated inhabitants to move to different parts of the world. As of 2015, the total refugees and people in refugee like situations in Africa were estimated at around four million (UNDESA, cited in de Haas, 2017). However, by the end of 2017 the figure has risen to over seven million with about a two million additions (UNHCR, 2018). The total population of concern (which includes refugees, people in refugee-like situations, IDPs and Stateless persons) was estimated at over 24 million (see Table 1). This number is second in rank when compared with other world regions, the first being Asia with an over 31 million total population of concern. The rise in refugee population is attributed to armed conflicts in Libya (North Africa), Nigeria and Mali (West Africa), the Central African Republic (Central Africa), South Sudan, and Eritrea (East Africa) (de Hass, 2017).
North Africa: Historically, North Africa (with particular reference to the Maghreb region consisting of Morocco, Algeria, Tunisia and Libya) is described as "Africa's emigration region par excellence" and noted for higher significant numbers of emigrants to Europe than any other African region (Flahaux & De Haas, 2016, p. 10). As of 2017, the total stock of North Africans living outside Africa was estimated as 9.7 million. This figure represents 86.8% of the total emigration stock in the region (UNCTAD, 2018). It is estimated that one of two Africans living abroad is a North African (European Commission, 2018b). Aside from being sources of migration, Maghreb countries also serve as transit region for sub-Saharan Africans aiming for Europe (Malka, 2018). This flow is often attributed to the region's geographical nearness to Europe, its labor treaties with many European nations, together with its colonial and post-colonial connections with France (UNCTAD, 2018;Natter, 2014). In addition, the relatively lower income and high unemployment rates in North African countries have spurred the frequent migration to Europe and other destination countries (IOM, 2018). The political and economic conflicts in the Maghreb which include the Tunisian Revolution (2010-2011) and the Libyan Crisis (2011 till present) coupled with the neighboring Egyptian Crisis (2011-2014) have led to serious public violence, displacement and societal breakdown impelling irregular migration to Europe and other nearby regions (Cummings, Pacitto, Lauro & Foresti, 2015;Fargues & Bonfanti, 2014). For example, the net migration figure of Libya is estimated as-100,338, making the country rank highest as nation of emigration not immigration (UNDESA, cited in African Union, 2017b). Over 90% of migrants on Libyan routes plan to migrate to Europe because of her long border sharing with the sub-Saharan region (Karagueuzian & Verdier-Chouchane, 2014). Current data on asylum applications to European Countries between September 2017 and December 2018 show that there were 11,850 applications from Algerians, 9480 from Moroccans, and 5460 from Egyptians (Eurostat, 2019). West Africa: Although 84% of migration within West African is internal, migrants originating from the region also target Europe as final destination (African Union, 2017a). Though North Africans are Africa's migration giant to Europe, data also show that the number of West African migrants to Europe is also on the increase (Flahaux & De Haas, 2016). Recent data show that Nigerians (390,000), Senegalese (270,000), and Ghanaians (250,000) constitute the highest number of migrants from West Africa to the EU, Norway and Switzerland (Pew Research Centre, 2018). In particular, 45% of Senegalese emigration is towards Europe while migration to other African regions and continents range between 6 and 28% (Altai Consulting, 2015). Generally, 72% of migrants from sub-Saharan countries are hosted in four European countries, namely Portugal (360,000), Italy (370,000), France (980,000) and the UK (1.27 Million) (Pew Research Centre, 2018).
Italy and Spain are the main destination countries for Nigerian migrants whereas migrants from Senegal mostly move to France (African Union, 2017b). Principally, the drivers of migration in West Africa are economic hardship and unemployment (Horwood et al., 2018). Other drivers include political crises, armed conflict, generalized violence and violation of human rights (African Union, 2017b). Due to incessant conflicts in parts of Mali and Nigeria, the West African sub region harbors considerable number of refugees and internally displaced persons (IDPs). According to UNHCR (2018) statistics, the total population of concern in Nigeria and Mali is about 2.7 million and 232,282, respectively. These forced displacements have influenced the number of asylum applications in neighboring countries and the EU. For example, from September 2017 to December 2018, there were 29,625 new asylum applications to the EU from citizens of Nigeria, 16,780 from Guinea, 11,220 from Ivory Coast, 8480 from Mali, 6545 from Cameroon, and 7115 from Senegal (Eurostat, 2019). Data shows that migrants from West African countries constitute 61% of all asylum applications in Italy while applicants from Nigeria top the list (OECD/ILO/IOM/UNHCR, 2018). It is documented that migration from West Africa to Europe is mostly irregular (African Union, 2017b).
Central Africa: The economic and political instabilities in this region which mainly originate from the Central African Republic (CAR) and the Democratic Republic of Congo (DRC) remain the principal drivers of migration. The number of migrants from Central Africa is estimated as 4.1 million out of which the majority is being hosted within the region by Gabon (Devillard, Bacchi, & Noack, 2015). Only Cameroon has a significant number of migrants (46%) residing in Europe with Italy and France as major destination countries (European Commission, JRC, 2018). Total population of concern was estimated at almost 1.5 million for CAR and 5.5 million DRC (UNHCR, 2018).
East Africa: Besides economic mishaps, migration in the East Africa subregion has majorly been propelled by political strives and armed conflicts in Somalia, South Sudan, and Eritrea (de Haas, 2017;Horwood et al., 2018). UNHCR total populations of concern in Somalia, South Sudan, and Sudan were estimated as 3.2 million, 4.3 million, and 3.1 million, respectively. Migrants from East Africa constitute 27% (9.8 million) of all migrants from Africa with a majority residing in other African countries such as Ethiopia, Kenya, Sudan, and Uganda (European Commission, JRC, 2018). Migration out of the region is less towards Europe but exhibits a major flow to Southern Africa and the Gulf States due to geographical proximity (European Commission, JRC, 2018).

Irregular Migration into Europe
People migrate through regular and irregular means. Regular migration involves meeting the requirements for entry into the countries of destination. This entails obtaining visa, residence, study or work permits. On the other hand, irregular migration consists in moving to another country via unofficial means. It is noncompliance to the migration regulations of origin, transit and destination countries, which often involves the assistance of smugglers and traffickers (Abebe, 2017;IOM, 2013). Following stricter regulations regarding African migration in the mid-1990s, the EU had witnessed irregular entry into its borders (Adepoju, 2011;African Union, 2017b). The stiffness of EU laws on migration has necessitated Africans to source for informal, irregular and underground ways of reaching Europe. Other factors implicated in irregular migration are lack of appropriate travel documents and financial means for regular migration (Giménez-Gómez, Walle, & Zergawu, 2017).
Many African migrants are noted for unauthorized migration into Europe through engaging in dangerous journeys across the Maghreb borders (Giménez-Gómez et al., 2017;Schoumaker et al., 2013). In addition to traveling through the sea by boat, many migrants gain access to Europe through hiding in ferries and vehicles, use of tourist visas or fake documents (Arnold, 2012). As patrols are intensified by border police along the dangerous routes, migrants discover and change to more complex routes to achieve their dreams of entering the European continent (Adepoju, 2011). Border controls may pose significant challenges to migrants, but African migrants are determined to reach European soil at any cost given the huge physical and financial resources expended on the journey (Arnold, 2012). However, the trend of irregular migration to Europe through the sea has declined in recent times with less than 200,000 crossings in 2018 as compared to more than 1.8 million recorded in 2015 (Frontex, 2019). This new trend is attributed to stricter laws which require return of illegal immigrants to transit origins before they land on European shores (BBC, 2019;Gros, 2018).
Since the end of the twentieth century, nearly 60,000 migrants have perished worldwide on migration routes. In 2017 alone, about 6280 missing/dead migrants were recorded out of which more than 50% occurred along the Mediterranean with majority of victims originating from sub-Saharan Africa (Migration Data Portal, 2018). Although, African migrants who gain (unauthorized) access into Europe via the sea have been the focus of media and the public, migration officials have suggested that visa over-stayers or those whose permits have expired constitute a larger percentage of illegal immigrants in the EU (Orrenius & Zavodny, 2016). Nevertheless, this pattern may change considering the rise in asylum applicants gaining entrance into the EU through land and sea borders (Orrenius & Zavodny, 2016).
Statistics on numbers of irregular migrants in the EU are imprecise, unreliable and usually of a guesstimate given its clandestine nature and outside of state control (Reed, 2018;Vespe, Natale, & Pappalardo, 2017). The population of the world's irregular migrants was estimated at around 50 million in 2009 (IOM, 2018). Also, for 2009, the Final Project Report of Project Clandestino provided an estimate ranging from 1.9 million to 3.8 million irregular migrants in Europe (European Commission, 2009). These figures seem to be lower as compared to the 2.4-5.5 million irregular migrants recorded in 2005 within the EU-25 (Kovacheva & Vogel, 2009). More recent data covering between 2008 and 2017 estimate 7.4 million illegal third-country nationals in the EU (European Commission, JRC, 2018). Specifically, the United Kingdom is estimated to have the highest number of irregular migrants with 417,000-863,000, followed by Germany (180,000-520,000), Italy (279,000-461,000), and Spain (354,000) (IOM, 2018). According to IOM (2017)

Demographics of Irregular Migrants
Generally, the percentage of African migrants is almost equal for males (52.9) and females (47.1) with a median age of 30.9 (UNDESA, 2017). Migrants from Africa are also likely to be more educated and have more socio-economic power than nonmigrants from the region (Lucas, cited in Schoumaker et al., 2013). However, the characteristics of irregular migrants tend to be mixed, consisting mainly of adult men and about 11-14% of children and women (Frontex, 2015) with less education. This trend is changing as there are increasing numbers of women and children among asylum seekers (Orrenius & Zavodny, 2016). More recently, 18% of women were recorded in illegal border crossing (Frontex, 2019). In 2015, Sweden recorded up to 43% of children asylum seekers out of which 50% were unaccompanied minors (Parusel, 2016). According to the European Commission (2018a), Germany has the highest number of unaccompanied minors between 2008 and 2017 (82,400), followed by Sweden (61,300) and Italy (26,435). Typically, irregular migrants have lower levels of education and tend to engage in informal low-skilled employment in country of origin (Orrenius & Zavodny, 2016). Given that African migrants have mixed motivations for crossing European borders by sea, it may be difficult to distinguish whether they are politically motivated (e.g., armed conflict and persecution) or economic migrants (e.g., poverty and unemployment); although most of them are expected to claim asylum (Giménez-Gómez et al., 2017).
In a sample of 1031 migrants (mostly irregular) who arrived in Italy by sea through Libya, 83% were found to be males (with thus only 17% females) and an average age of 27 (IOM, 2016). About 66% were single, while 30% were married. Two thirds of the sample were of West African origin (with Nigerians topping the list), the majority were asylum seekers (61%), whereas 25% had no legal status. Ivory Coast (36%) had the highest number of females followed by Nigeria (26%) and Eritrea (22%). Most of the migrants lack formal education with only 3% having completed university education. Men were more educated than women with nearly 90% of women yet to complete lower secondary education. Twenty-five percent of migrants were employed in skilled manual works which include metal and construction workers, tailors and mechanics. Another 21% were trained in unskilled manual labor such as waiters, cleaners, unskilled construction workers and drivers. Similarly, almost the same percentage was employed in the agricultural sector.

Globalization and Migration
Globalization plays a pivotal role in modern migration trends. The era of globalization has propelled the movement of migrants within and outside the African continent (African Union, 2017a). Globalization is the integration of the world's economies, anchored by movement of people, capital, trade, and knowledge/technological diffusion across international borders (International Monetary Fund, 2000a, 2000b. Given that trade, capital and technology cannot in themselves move across borders without human mobility, intra and extra-continental migration then becomes inevitable. The interconnectedness and interdependence of the modern world has removed barriers and no country is an "island" any longer (Barriga, 2013). Globalization has necessitated the birth of many world unions such as the African Union, European Union, OECD and others in order to maximize the benefits and minimize costs. Globalization impels migration and sets millions of people in motion while migration influences the intensification of political and socioeconomic relations across countries. These pose serious challenges to the social and political decision-making of nation-states at effectively regulating migration flows across their national boundaries (Dokos, 2017).
One aspect of globalization that has spurred international migration is the evolution of telecommunication and transportation technologies (IOM, 2018). The development of jet engines in the mid-1960s has dramatically aided the durability of air transportation by enabling planes to reach very far destinations in limited hours at reasonably low cost, thereby increasing international migration flows of people and goods within and across regions (Hoovestal, 2013). The "death of distance" has been orchestrated by decreasing costs of air travel, ocean freight, international telephoning and satellite charges (Cairncross, 1997). These developments have increased and expanded the trend of regular and legal migration worldwide (IOM, 2018). Notably, the development of modern means of communication has aided international migration. The dramatic rise in the use of smartphones with accompanied internet technology has enabled millions of people from both developed and developing nations to have access to different social media platforms such as Facebook, WhatsApp, Instagram, Skype, Zoom, and the like. These developments have increased social networks among people internationally and moved the different parts of the world closer together.
The availability of modern telecommunication has aided the interconnectivity between migrants and their families and friends in countries of origin. These interconnections provide information to family members on movement of migrants throughout the journey and facilitate the planning of the union of family members left behind as soon as the migrant reaches the country of destination (IOM, 2018). The advancement of smart phone technology has assisted refugees and irregular migrants to access information on the financial cost of migration, safest routes into transit and destination countries via connectivity with smugglers (Triandafyllidou & Maroukis, 2012). For example, there are many specialized apps such as InfoAid, Refugermany, and Arriving in Belin which provide connectivity and information on safer and perilous pathways to crossing the border to Europe. In fact, transit to Greece in 2015 through the Eastern Mediterranean route has been attributed to the aids of such mobile connectivity (McAuliffe, 2016). On reaching destination countries, these communication technologies enable migrants to navigate their ways and get remotely connected with other migrants of similar origin who provide information on systems of operations and integration within communities in host countries.
Through modern communication systems, potential migrants are able to know the rights to claim regarding asylum applications based on EU regulations and the kinds of documents needed for such application before departing from the country of origin (Kugiel, 2016).

The Costs and Benefits of Migration
Given that the flow of extra-continental migration seems unbalanced and usually from African countries to the European Union, it is important to examine the benefits and costs of migration to both sides. Despite leaving Africa for Europe for better lives, migrants continue to have strong social, cultural and economic ties to their country of origin. These are accomplished by sending goods and remittances home for reduction of poverty and increase in the standard of living of family members. Remittances are utilized for investment in children's education, purchase of land, building houses, loan repayment and catering for health care costs of family members (Nwana, 2014;Thouez, 2004). After experiencing a shortfall in 2015 and 2016, the global remittance to low-and middle-income nations increased by 8.5% to $US466 million in 2017 (World Bank, 2018). Nigeria ($US22 billion) and Egypt ($US20 billion), respectively, occupy 5th and 6th positions as countries receiving huge remittances in the world. Also, in the same year, the remittances to sub-Saharan Africa increased by 11.4% to $US38 billion. Aside from Nigeria, the two other countries in sub-Saharan Africa with largest remittances are Senegal ($US2.2 billion) and Ghana ($US2.2 billion). The total value of remittance to low-and-middle-income countries is shown to surpass what they received as official aids from the developed world (Kugiel, 2016;World Bank, 2016). Specifically, the remittance value of Nigerians living in Europe is reported to surpass the value of EU aids to the Nigerian government by ten times (Scazzeri & Sringford, 2017). Remittance constitute up to 50-90% of the household income in some developing countries (Kunz, 2008).
There also is empirical evidence demonstrating strong and positive nexus between remittances from host countries and poverty decline in developing countries with reduction in child mortality by 16-fold after migrants move abroad (Tsaurai, 2018;World Bank, 2016). In addition, there is transfer of skills and knowledge acquired in the host countries to countries of origin when migrants return home for short, long and permanent stays (Arhin-Sam, 2019). However, it is argued that remittance may create further problems for departure countries in terms of inflation, overvalued exchange rate and increased import if remittances are used for consumption purposes instead of investments (Bruni, 2017).
Although positive feedback effects of migration exist for countries of origin, there are also some drawbacks, which may have significant negative repercussions. One of these is "brain drain"-a situation of flight of skilled human resources. With an increase in overall migration flow of skilled working-age groups from Africa to developed nations in Europe and North America, Africa stands the risks of losing her young and skillful population who are expected to drive innovations and developments. Already, brain drain is reported to be acute in sub-Saharan Africa. Brain drain can weaken business growth, limit innovation and national development in technology, agriculture, health and education (Agopyan et al., 2013). Many small sub-Saharan countries are reported to lose a substantial part of their college graduates to the developed world (Docquier, 2014). Countries of origin are at greater loss when newly trained professionals such as medical doctors depart for another country without utilizing their skills in home countries. In 2006, Liberia, Ghana and Uganda respectively lost 43%, 30% and 20% of their medical doctors to Canada and the United States (Bach, 2006). In this instance, country of origin's return on investments for such training is lost but gained by destination countries. For example, developing countries suffer a total loss of $US60 billion on educational investment alone at the migration of over 3 million highly skilled populations to OECD countries in 2004 (Stalker, cited in Thouez, 2004). Campaigns by, e.g., the Council of Graduate Schools, more or less desperately call for "brain circulation" (Boehnke, 2013).
Despite huge migration flows, receiving countries have been described as top gainers in the migration process given the migrants' economic contributions. There is consensus on the positive economic impact of migrants in destination countries (Bruni, 2017). For example, the 6.6 million immigrants in Germany contributed an individual net gain of e3300 amounting to e22 billion in 2012 (Bonin, 2015). According to Bonin, this trend will continue throughout migrants' lives in which they will receive less in transfer payments than what they have paid into the German State. Additionally, there is an increased savings for taxpayers because destination countries may not need to contribute to the education development of migrants which had been paid for in countries of origin (Dustman & Frattini, 2014). In all, migration will positively impact the work-age population in Europe which is predicted to suffer serious decline within the next 30 years (UNDESA, 2013).
The negative side of migration to receiving countries is higher competition for employment and problems constituted by irregular immigrants. It is assumed that irregular immigrants are less likely to be innovative in their activities because they are not trained in science, technology, engineering and math (STEM). Irregular migrants are likely to engage in entrepreneurial activities which are small and informal, and most of the time, their incomes are not spent or saved in Europe, but remitted to their countries of origin (Orrenius & Zavodny, 2016). Remittances benefit countries of origin but reduce the benefits of migration to hosting countries (Rainer, Straubhaar, Vadean, & Vadean, 2006). On the other hand, irregular and even regular migration enables firms to have access to cheap or reduced labor cost and volunteers for jobs which natives consider as dangerous, difficult, seasonal, dirty and low-paid domestic service jobs (Rainer et al., 2006; see also Wax and Richwine in the Philadelphia Inquirer). Migration increases competition for low-skilled jobs which may have adverse effect on lowly skilled natives' employment rate and prior immigrants. However, this effect is predicted to dissipate gradually as the economy adjusts (Orrenius & Zavodny, 2016). World Bank. (2016). Annual report. Retrieved from http://documents.worldbank.org/curated/ en/782691475489708512/World-Bank-annual-report-2016-organizational-information-and-len ding-data-appendixes. World Bank. (2018). Annual report. Retrieved from http://documents.worldbank.org/curated/en/ 630671538158537244/The-World-Bank-Annual-Report-2018. Worldometers. (2019). Nigeria population (live). Retrieved from https://www.worldometers.info/ world-population/nigeria-population/.
Open Access This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made. The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.