Question: Halfdan Ragnarsson was a commander of a coalition of Norse warriors, originating from where?

full_context: Document 1: Hvitserk: Hvitserk ("Hvitsärk") "White-Shirt" was one of the sons of the legendary 9th-century Norsemen Ragnar Lothbrok and his wife Aslaug. He is attested to by the Tale of Ragnar's Sons ("Ragnarssona þáttr"). He is not mentioned in any source which mentions Halfdan Ragnarsson, one of the leaders of the Great Heathen Army which invaded the Kingdom of East Anglia in 865, or vice versa which consequently led some scholars to suggest that they are the same individual with "Hvitserk" being only a nickname.
Document 2: Vikings: Vikings (Old English: "wicing"—"pirate", Danish and Bokmål: "vikinger" ; Swedish and Nynorsk: "vikingar" ; Icelandic: "víkingar" , from Old Norse "víkingar "), were Norse seafarers, mainly speaking the Old Norse language, who raided and traded from their Northern European homelands across wide areas of northern, central and eastern Europe, during the late 8th to late 11th centuries. The term is also commonly extended in modern English and other vernaculars to the inhabitants of Viking home communities during what has become known as the Viking Age. This period of Nordic military, mercantile and demographic expansion constitutes an important element in the early medieval history of Scandinavia, Estonia, the British Isles, France, Kievan Rus' and Sicily.
Document 3: Early Scandinavian Dublin: The First Viking Age in Ireland began in 795, when Vikings began carrying out hit-and-run raids on Gaelic Irish coastal settlements. Over the following decades the raiding parties became bigger and better organized; inland settlements were targeted as well as coastal ones; and the raiders built naval encampments known as longphorts to allow them to remain in Ireland throughout the winter. In the mid 9th century, Viking leader Turgeis or "Thorgest" founded a stronghold at Dublin, plundered Leinster and Meath, and raided other parts of Ireland. He was killed by the High King, Máel Sechnaill mac Máele Ruanaid, which was followed by several Irish victories against the Vikings and the seizure of Dublin in 849. Shortly after, a new group of Vikings known as the Dubgaill ("dark foreigners") came to Ireland and clashed with the earlier Viking settlers, now called the Finngaill ("fair foreigners"). The wavering fortunes of these three groups and their shifting alliances, together with the shortcomings of contemporary records and the inaccuracy of later accounts, make this period one of the most complicated and least understood in the fledgling city's history. In 853 a Viking warlord called Amlaíb (, possibly Olaf the White) arrived and made himself king of Dublin. He ruled along with his brothers Ímar ("Ívarr", possibly Ivar the Boneless) and Auisle ("Ásl"). For the next fifteen years or so, they used Dublin as their base for a series of campaigns against Irish kingdoms. During these conflicts they briefly allied themselves with several Irish kings. The Dublin Vikings also carried out a number of raids in Britain at this time. The deaths of Ivar (c.873) and Olaf (c.874) were followed by internecine conflict among the Vikings. Although intermittent warfare between the Vikings and the Irish continued, these inner conflicts weakened the Viking colonies and made it easier for the Irish to unite against them. In 902, Cerball mac Muirecáin, king of Leinster, and Máel Findia mac Flannacáin, king of Brega, launched a two-pronged attack on Dublin and drove the Vikings from the city. However, in 914 the Vikings now known as the Uí Ímair (House of Ivar) would return to Ireland, marking the beginning of the Second Viking Age.
Document 4: Great Heathen Army: The Great Viking Army or Great Danish Army, known by the Anglo-Saxons as the Great Heathen Army (OE: "mycel hæþen here"), was a coalition of Norse warriors, originating from Denmark (and likely also from Sweden and Norway) who came together under a unified command to invade the four Anglo-Saxon kingdoms that constituted England in AD 865.
Document 5: Halfdan Ragnarsson: Halfdan Ragnarsson (Old Norse: "Hálfdan" ; or "Healfdene"; Old Irish: "Albann" ; died 877) was a Viking leader and a commander of the Great Heathen Army which invaded the Anglo-Saxon kingdoms of England, starting in 865. According to the tradition recorded in the Norse sagas he was one of the sons of Ragnar Lodbrok, and his brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye and Ubba. He was the first Viking King of Northumbria and a pretender to the throne of Kingdom of Dublin. He died at the Battle of Strangford Lough in 877 trying to press his Irish claim.
Document 6: Ivar the Boneless: Ivar the Boneless (Old Norse: "Ívarr hinn Beinlausi" ; ) was a Viking leader and a commander who invaded what is now England. According to the "Tale of Ragnar Lodbrok", he was the son of Ragnar Lodbrok and Aslaug. His brothers included Björn Ironside, Halfdan Ragnarsson, Hvitserk, Sigurd Snake-in-the-Eye and Ubba.
Document 7: Ragnvald Heidumhære: Ragnvald Heidumhære (or Rognvald) was a legendary, possibly historic, petty king or chieftain of Vestfold in what is today Norway in the 9th century, according to "Ynglingatal" and to "Ynglinga saga" in "Heimskringla". He was the son of Ragnar Lodbrok or Olaf Geirstad-Alf. His name "Heiðumhæri" could be translated as "highly honoured"
Document 8: Halfdan the Black: Halfdan the Black (Old Norse: "Halfdanr Svarti") (  810 –  860 ) was a ninth-century king of Vestfold. He belonged to the House of Yngling and was the father of Harald Fairhair, the first king of Norway.
Document 9: Danes (Germanic tribe): The Danes were a North Germanic tribe inhabiting southern Scandinavia, including the area now comprising Denmark proper, during the Nordic Iron Age and the Viking Age. They founded what became the Kingdom of Denmark. The name of their realm is believed to mean "Danish March", viz. "the march of the Danes" in Old Low German, referring to their southern border zone between the Eider and Schlei rivers.
Document 10: Battle of Strangford Lough: The Battle of Strangford Lough was fought in 877 between two groups of rival Vikings described by the Irish Annals as the "fair heathens" and the "dark heathens". The "Annals of Ulster" describe "Albann", a figure usually identified with Halfdan Ragnarsson, a leader of the Great Heathen Army, as king of the "dark heathens", and "Cogad Gáedel re Gallaib" identifies Bárid mac Ímair, King of Dublin as the leader of the "fair heathens". All accounts agree Halfdan was killed in the battle, and "Cogad Gáedel re Gallaib" adds that Bárid was wounded in it.
Document 11: Anglo-Saxons: The Anglo-Saxons were a people who inhabited Great Britain from the 5th century. They comprise people from Germanic tribes who migrated to the island from continental Europe, their descendants, and indigenous British groups who adopted some aspects of Anglo-Saxon culture and language. Historically, the Anglo-Saxon period denotes the period in Britain between about 450 and 1066, after their initial settlement and up until the Norman conquest.
Document 12: Ubba: Ubba, also known as Hubba, Ubbe, and Ubbi, was a mid-ninth-century Viking chieftain and one of the commanders of the Great Army, a coalition of Norse warriors that in AD 865 invaded the Anglo-Saxon kingdoms of Northumbria, Mercia, East Anglia and Wessex.
Document 13: Ímar: Ímar (Old Norse: "Ívarr" ; died c. 873) was a Viking leader in Ireland and Scotland in the mid-late ninth century who founded the Uí Ímair dynasty, and whose descendants would go on to dominate the Irish Sea region for several centuries. He was the son of the king of Lochlann, identified in the non-contemporary "Fragmentary Annals of Ireland" as Gofraid. The "Fragmentary Annals" name Auisle and Amlaíb Conung as his brothers. Another Viking leader, Halfdan Ragnarsson, is considered by some scholars to be another brother. The Irish Annals title Amlaíb, Ímar and Auisle "kings of the foreigners". Modern scholars use the title "kings of Dublin" after the Viking settlement which formed the base of their power. Some scholars consider Ímar to be identical to Ivar the Boneless, a Viking commander of the Great Heathen Army named in contemporary English sources who also appears in the Icelandic sagas as a son of the legendary Viking Ragnar Lodbrok.
Document 14: The Strongbow Saga: The Strongbow Saga is a historical fiction series by Judson Roberts. The Saga tells the story of Halfdan Hroriksson, a young Viking trying to avenge his brother's murder. The series weaves the story of the fictional Halfdan into the lives of actual historical figures of the period, including famous Viking leaders Ragnar Lodbrok, Ivar the Boneless, Bjorn Ironside, and Hastein, as well as Frankish leaders King Charles the Bald and Count Robert the Strong. Actual historical events form a backdrop for parts of the story, including a Danish attack up the Seine River in the year 845 which was reported in several contemporary Frankish sources, including the Annals of St. Bertin.
Document 15: Saxons: The Saxons (Latin: "Saxones" , Old English: "Seaxe" , Old Saxon: "Sahson" , Low German: "Sassen" , German: "Sachsen" , Dutch: "Saksen" , Welsh: "Saeson" ) were a group of Germanic tribes first mentioned as living near the North Sea coast of what is now Germany (Old Saxony), in the late Roman empire. They were soon mentioned as raiding and settling in many North Sea areas, as well as pushing south inland towards the Franks. Significant numbers settled in large parts of Great Britain in the early Middle Ages and formed part of the merged group of Anglo-Saxons who eventually organised the first united Kingdom of England. Many Saxons however remained in Germania (Old Saxony c. 531-804), where they resisted the expanding Frankish Empire through the leadership of the semi-legendary Saxon hero, Widukind. Initially, Saxons of Britain and those of Old Saxony (Northern Germany) were both referred to as 'Saxons' in an indiscriminate manner. The term Anglo-Saxon, in turn, came into practice in the 8th century (probably by Paul the Deacon) to distinguish English Saxons from continental Saxons (Ealdseaxe, 'old Saxons').
ground_truth: Denmark