Question: Who was the father of the Danish king involved in the Battle of Marton?

Context paragraphs:
Canute IV of Denmark: Canute IV (  1042  – 10 July 1086), later known as Canute the Holy (Danish: "Knud IV den Hellige" ) or Saint Canute ("Sankt Knud"), was King of Denmark from 1080 until 1086.  Canute was an ambitious king who sought to strengthen the Danish monarchy, devotedly supported the Roman Catholic Church, and had designs on the English throne.  Slain by rebels in 1086, he was the first Danish king to be canonized.  He was recognized by the Roman Catholic Church as patron saint of Denmark in 1101.

Alexander (satrap): Alexander (in Greek Αλέξανδρος; died 220 BC) was brother of Molon.  On the accession of the Seleucid king Antiochus III, afterwards called the Great, in 223 BC, he entrusted Alexander with the government of the satrapy of Persis and Molon received Media.  Antiochus was then only fifteen years of age, and this circumstance together with the fact that Hermeias, a crafty intriguer whom every one had to fear, was all-powerful at his court, induced the two brothers to form the plan of causing the upper satrapies of the kingdom to revolt.  It seems to have been the secret wish of Hermeias to see the king involved as many difficulties as possible, and it was on his advice that the war against the rebels was entrusted to men without courage and ability.  In 220 BC, however, Antiochus himself undertook the command.  Molon was deserted by his troops, and to avoid falling into the hands of the king, put an end to his own life.  All the leaders of the rebellion followed his example, and one of them, who escaped to Persis, killed Molon's mother and children, persuaded Alexander to put an end to his life, and at last killed himself upon the bodies of his friends.

Gurre-Lieder: Gurre-Lieder is a large cantata for five vocal soloists, narrator, chorus and large orchestra, composed by Arnold Schönberg, on poems by the Danish novelist Jens Peter Jacobsen (translated from Danish to German by Robert Franz Arnold).  The title means "songs of Gurre", referring to Gurre Castle in Denmark, scene of the medieval love-tragedy (related in Jacobsen's poems) revolving around the Danish national legend of the love of the Danish king Valdemar Atterdag (Valdemar IV, 1320–1375, spelt Waldemar by Schönberg) for his mistress Tove, and her subsequent murder by Valdemar's jealous Queen Helvig (a legend which is historically more likely connected with his ancestor Valdemar I).

Danish March: The terms Danish March and March of Schleswig (German: "Dänische Mark" or "Mark Schleswig ") are used to refer to a territory in modern-day Schleswig-Holstein north of the Eider and south of the Danevirke.  It was established in the early Middle Ages as a March of the Frankish Empire to defend against the Danes.  The term "Danish March" is a modern designation not found in mediaeval sources.  According to the "Royal Frankish Annals" the Danish King led his troops "into the March" in 828 ("ad marcam ").  In the 852 Yearbook of Fulda there is mention of a "Guardian of the Danish Border" ("custodes Danici limitis ").

Hemming Halfdansson: Hemming Halfdansson (died 837) was "of the Danish race, a most Christian leader".  He was probably a son of Halfdan, a leading Dane who became a vassal of Charlemagne in 807.  He was probably related to the Danish royal family, as "Hemming" was one of their favoured names.  The onomastic evidence includes the Danish king Hemming I (reigned 810–12) and then a Hemming II, who was recalled to Denmark from Francia by his brothers Harald Klak and Reginfrid after Hemming I's death.  This Hemming was probably the same person as Hemming Halfdansson.  He probably soon returned to Francia, since there is no evidence of him in Danish politics after he and his brothers were driven out by the sons of Godfrid in 813.

Battle of Visby: The Battle of Visby was fought in 1361 near the town of Visby on the island of Gotland, between the forces of the Danish king and the Gutnish country yeomen.  The Danish force was victorious.

Bjarkamál: Bjarkamál (Bjarkemål in modern Norwegian and Danish) is an Old Norse poem from around the year 1000.  Only a few lines have survived in the Icelandic version, the rest is known from Saxo's version in Latin.  The latter consists of 298 hexameters, and tells the tale of Rolf Krake's downfall at Lejre on the isle of Sjælland, described in a dialogue between two of Rolf Krake's twelve berserkers, Bodvar Bjarke (hence the name of the poem), the most famous warrior at the court of the legendary Danish king Rolf Krake, and Hjalte (= hilt).  The poem opens with Hjalte waking up his fellow berserkers, having realized they are under attack.  In 1030, King Olav had the bard Tormod Kolbrunarskald recite the "Bjarkamál" to rouse his outnumbered army in the morning before the start of the Battle of Stiklestad, according to "Fóstbrœðra saga".

Siege of Tallinn: Victory over the Swedish army in the Battle of Lihula inspired the Oeselians for further fightings.  In 1221 they tried to conquer the Danish stronghold in Tallinn with the help of Revalians, Harrians and Vironians.  They besieged the stronghold for 14 days and all Danish outbreaks were repulsed.  One day four cogs appeared unexpectedly, which the Oeselians thought to have been carrying the Royal Army of the Danish king.  The siege was ended and the Oeselians left.

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.

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.

Supporting paragraphs: Battle of Marton Halfdan Ragnarsson
Answer: Ragnar Lodbrok