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

Context paragraphs:
Battle of Reading (871): The first Battle of Reading was a battle on 4 January 871 at Reading in what is now the English county of Berkshire.  It was one of a series of battles, with honours to both sides, that took place following an invasion of the then kingdom of Wessex by an army of Danes led by Bagsecg and Halfdan Ragnarsson in an attempt to conquer Wessex.  Both battle and campaign are described in the "Anglo-Saxon Chronicle", and this account provides the earliest known written record of the existence of the town of Reading.

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.

Auisle: Auisle or Óisle (Old Norse: "Ásl "or" Auðgísl" ; died c. 867) was a Viking leader in Ireland and Scotland in the mid-late ninth century.  He was the son of the king of Lochlann, identified in the non-contemporary "Fragmentary Annals of Ireland" as Gofraid, and brother of Amlaíb Conung and Ímar, the latter of whom founded the Uí Ímair dynasty, and whose descendants would go on to dominate the Irish Sea region for several centuries.  Another Viking leader, Halfdan Ragnarsson, is sometimes considered a brother.  The Irish Annals title Auisle, Ímar and Amlaíb "kings of the foreigners".  Modern scholars use the title "kings of Dublin" after the Viking settlement which formed the base of their power.

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.

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.

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.

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.

Amlaíb Conung: Amlaíb Conung (Old Norse: "Óláfr" ; died c. 874) was a Viking leader in Ireland and Scotland in the mid-late ninth century.  He was the son of the king of Lochlann, identified in the non-contemporary "Fragmentary Annals of Ireland" as Gofraid, and brother of Auisle and Ímar, the latter of whom founded the Uí Ímair dynasty, and whose descendants would go on to dominate the Irish Sea region for several centuries.  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.  The epithet "Conung" is derived from the Old Norse "konungr" and simply means "king".  Some scholars consider Amlaíb to be identical to Olaf the White, a Viking sea-king who features in the "Landnámabók" and other Icelandic sagas.

Battle of Marton: The Battle of Marton or Meretum took place on approximately 22 March 871 at a place recorded as Marton, perhaps in Wiltshire or Dorset.  According to the Anglo-Saxon Chronicle, the battle took place "about two months" after a prior loss to the Danes at the Battle of Basing, in which both sides suffered heavy losses.  King Æthelred of Wessex and Prince Alfred fought against the Danes, who were possibly under the leadership of King Halfdan Ragnarsson, who had commanded many of the earlier battles that year.  The Chronicle tells us that the forces were split in two and that the Danes were initially put to flight, but at the end of the day and after "much slaughter" the Danes remained masters of the field.  Among the fallen was Bishop Heahmund.

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.

Supporting paragraphs: Halfdan Ragnarsson Great Heathen Army
Answer: Denmark