Question: In pre-Roman Gaul the Belgic tribe of the Veliocasses or Velocasses controlled a large area in the lower Seine valley, which retains a trace of their name, as the Vexin, in 52 they raised 3000 men to support which son of Celtillus the Avernian, leader of the Gallic tribes?
Context: The Segni (sometimes Segui) were a tribe living in Belgic Gaul when Julius Caesar's Roman forces entered the area in 57 BCE.  They are known from his account of the "Gallic War".  They were one of a group of tribes listed by his local informants as the "Germani" of Belgian Gaul, along with the Eburones, Condrusi, Paemani (or Caemani), and Caeroesi (or Caeraesi).  The Segni do not appear in the first listing of the Germani, which was a listing of Germani sending men to fight Caesar.  But they appear in a later mention, after the defeat of the Eburones: The Segui and Condrusi, of the nation and number of the Germans ["Germani"], and who are between the Eburones and the Treviri, sent embassadors to Caesar to entreat that he would not regard them in the number of his enemies, nor consider that the cause of all the Germans on this side the Rhine was one and the same; that they had formed no plans of war, and had sent no auxiliaries to Ambiorix.  Caesar, having ascertained this fact by an examination of his prisoners, commanded that if any of the Eburones in their flight had repaired to them, they should be sent back to him; he assures them that if they did that, he will not injure their territories. Aulerci is a generic name for some of the Celtic peoples of ancient Gaul, which included several Celtic tribes.  Julius Caesar ("B. G." ii.  34) names the Aulerci with the Veneti and the other maritime states.  In "B. G." vii.  75, he enumerates, among the clients of the Aedui, the Aulerci Brannovices and Brannovii, as the common text stands; but the names in this chapter of Caesar are corrupt, and Brannovii does not appear to be genuine.  If the name Aulerci Brannovices is genuine in vii.  75, this branch of the Aulerci, which was dependent on the Aedui, must be distinguished from those Aulerci who were situated between the Lower Seine and the Loire, and separated from the Aedui by the Senones, Carnutes, and Bituriges Cubi. The Battle of Alesia or Siege of Alesia was a military engagement in the Gallic Wars that took place in September, 52 BC, around the Gallic "oppidum" (fortified settlement) of Alesia, a major centre of the Mandubii tribe.  It was fought by the army of Julius Caesar against a confederation of Gallic tribes united under the leadership of Vercingetorix of the Arverni.  It was the last major engagement between Gauls and Romans, and is considered one of Caesar's greatest military achievements and a classic example of siege warfare and investment.  The battle of Alesia marked the end of Gallic independence in France and Belgium. In pre-Roman Gaul the Belgic tribe of the Veliocasses or Velocasses controlled a large area in the lower Seine valley, which retains a trace of their name, as the Vexin.  According to Julius Caesar' "Commentary on the Gallic Wars" the Veliocasses participated in the tribal coalition of the Belgae that resisted the Romans in 57 BCE.  In 52 they raised 3000 men to support Vercingetorix, and fought alongside the Bellovaci in the final rebellion against Roman hegemony. The Menapii were a Belgic tribe of northern Gaul in pre-Roman and Roman times.  According to descriptions in such authors as Strabo, Caesar, Pliny the Elder and Ptolemy their territory had stretched northwards to the mouth of the Rhine in the north, but more lastingly it stretched along the west of the Schelde river.  In later geographical terms this territory corresponds roughly to the modern coast of Flanders, the Belgian provinces of East and West Vlaanderen.  It also extended into neighbouring France and the river deltas of the southern Netherlands. Rouen, France, was founded by the Gaulish tribe of Veliocasses, who controlled a large area in the lower Seine valley, which today retains a trace of their name as the Vexin.  The Gauls named the settlement "Ratumacos" and the Romans called it "Rotomagus".  Roman Rotomagus was the second city of Gallia Lugdunensis, after Lugdunum (Lyon).  After the reorganization of the empire by Diocletian, Rouen became the chief city of the divided province of Gallia Lugdunensis II and reached the peak of its Roman development, with an amphitheatre and "thermae", the foundations of which remain today.  In the 5th century, it became the seat of a bishopric and later a capital of Merovingian Neustria. The Gallic Wars were a series of military campaigns waged by the Roman proconsul Julius Caesar against several Gallic tribes.  Rome's war against the Gallic tribes lasted from 58 BC to 50 BC and culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the Roman Republic over the whole of Gaul (mainly present-day France and Belgium).  While militarily just as strong as the Romans, the internal division between the Gallic tribes helped ease victory for Caesar, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late.  The wars paved the way for Julius Caesar to become the sole ruler of the Roman Republic. Vercingetorix was the son of Celtillus the Avernian, leader of the Gallic tribes.  Vercingetorix came to power after his formal designation as chieftain of the Arverni at the oppidum Gergovia in 52 BC.  He immediately established an alliance with other Gallic tribes, took command and combined all forces, and led them in the Celts' most significant revolt against Roman power.  He won the Battle of Gergovia against Julius Caesar in which several thousands Romans and allies died and Caesar's Roman legions withdrew. This is a list of known wars, conflicts, battles/sieges, missions and operations involving ancient Greek city states and kingdoms, Magna Graecia, other Greek colonies (First Greek colonisation, Second Greek colonisation, Greeks in pre-Roman Crimea, Greeks in pre-Roman Gaul), Greek Kingdoms of Hellenistic period, Byzantine Empire/ Byzantine Greeks, Byzantine Greek successor states of the Byzantine Empire, Kingdom of Greece and Greece between 3000 BC and the present day. The Lexovii (Ληξόβιοι , Strabo; Ληξούβιοι , Ptol.  ii.  8.  § 2), were a Celtic people, on the coast of Gallia, immediately west of the mouth of the Seine.  When the Veneti and their neighbors were preparing for Julius Caesar's attack (56 BC), they applied for aid to the Osismii, Lexovii, Namnetes, and others.  ("B. G." iii.  9, 11.)  Caesar sent Quintus Titurius Sabinus against the Unelli, Curiosolites, and Lexovii, to prevent their joining the Veneti.  A few days after Sabinus reached the country of the Unelli, the Aulerci Eburovices and the Lexovii murdered their council or senate, as Caesar calls it, because they were against the war; and they joined Viridovix, the chief of the Unelli.  The Gallic confederates were defeated by Sabinus, and compelled to surrender.  ("B. G." iii.  17-19.)  The Lexovii took part in the great rising of the Galli against Caesar (52 BC); but their force was only 3000 men.  ("B. G." vii.  75.)  Walckenaer supposes that the territory of the Lexovii of Caesar and Ptolemy comprised both the territories of Lisieux and Bayeux, though there was a people in Bayeux named Baiocasses; and he further supposes that these Baiocasses and the Viducasses were dependent on the Lexovii, and within their territorial limits.  The capital of the Lexovii, or Civitas Lexoviorum, as it is called in the Notitia Dignitatum, is Lisieux, in the French department of Calvados, where the present-day inhabitants are still called "Lexoviens".  Under the Romans, the "oppidum" of the Lexovii was called "Noviomagus Lexoviorum", "Newfield of the Lexovii".  The country of the Lexovii was one of the parts of Gallia from where the passage to Britain was made.
Answer: Vercingetorix