Question: Who introduced a system of musical notation in the 14th century that is used in the area where most of the invasion of the eastern Roman Empire took place?

Knowledge 1: Middle Ages
As Western Europe witnessed the formation of new kingdoms, the Eastern Roman Empire remained intact and experienced an economic revival that lasted into the early 7th century. There were fewer invasions of the eastern section of the empire; most occurred in the Balkans. Peace with Persia, the traditional enemy of Rome, lasted throughout most of the 5th century. The Eastern Empire was marked by closer relations between the political state and Christian Church, with doctrinal matters assuming an importance in eastern politics that they did not have in Western Europe. Legal developments included the codification of Roman law; the first effort—the Theodosian Code—was completed in 438. Under Emperor Justinian (r. 527–565), another compilation took place—the Corpus Juris Civilis. Justinian also oversaw the construction of the Hagia Sophia in Constantinople and the reconquest of North Africa from the Vandals and Italy from the Ostrogoths, under Belisarius (d. 565). The conquest of Italy was not complete, as a deadly outbreak of plague in 542 led to the rest of Justinian's reign concentrating on defensive measures rather than further conquests. At the emperor's death, the Byzantines had control of most of Italy, North Africa, and a small foothold in southern Spain. Justinian's reconquests have been criticised by historians for overextending his realm and setting the stage for the Muslim conquests, but many of the difficulties faced by Justinian's successors were due not just to over-taxation to pay for his wars but to the essentially civilian nature of the empire, which made raising troops difficult. [SEP] Knowledge 2: Late Middle Ages
The main representatives of the new style, often referred to as ars nova as opposed to the ars antiqua, were the composers Philippe de Vitry and Guillaume de Machaut. In Italy, where the Provençal troubadours had also found refuge, the corresponding period goes under the name of trecento, and the leading composers were Giovanni da Cascia, Jacopo da Bologna and Francesco Landini. Prominent reformer of Orthodox Church music from the first half of 14th century was John Kukuzelis; he also introduced a system of notation widely used in the Balkans in the following centuries.
Knowledge 1: Palermo
As the Roman Empire was falling apart, Palermo fell under the control of several Germanic tribes. The first were the Vandals in 440 AD under the rule of their king Geiseric. The Vandals had occupied all the Roman provinces in North Africa by 455 establishing themselves as a significant force. They acquired Corsica, Sardinia and Sicily shortly afterwards. However, they soon lost these newly acquired possessions to the Ostrogoths. The Ostrogothic conquest under Theodoric the Great began in 488; Theodoric supported Roman culture and government unlike the Germanic Goths. The Gothic War took place between the Ostrogoths and the Eastern Roman Empire, also known as the Byzantine Empire. Sicily was the first part of Italy to be taken under control of General Belisarius who was commissioned by Eastern Emperor. Justinian I solidified his rule in the following years. [SEP] Knowledge 2: Late Middle Ages
The main representatives of the new style, often referred to as ars nova as opposed to the ars antiqua, were the composers Philippe de Vitry and Guillaume de Machaut. In Italy, where the Provençal troubadours had also found refuge, the corresponding period goes under the name of trecento, and the leading composers were Giovanni da Cascia, Jacopo da Bologna and Francesco Landini. Prominent reformer of Orthodox Church music from the first half of 14th century was John Kukuzelis; he also introduced a system of notation widely used in the Balkans in the following centuries.
Knowledge 1: Middle Ages
As Western Europe witnessed the formation of new kingdoms, the Eastern Roman Empire remained intact and experienced an economic revival that lasted into the early 7th century. There were fewer invasions of the eastern section of the empire; most occurred in the Balkans. Peace with Persia, the traditional enemy of Rome, lasted throughout most of the 5th century. The Eastern Empire was marked by closer relations between the political state and Christian Church, with doctrinal matters assuming an importance in eastern politics that they did not have in Western Europe. Legal developments included the codification of Roman law; the first effort—the Theodosian Code—was completed in 438. Under Emperor Justinian (r. 527–565), another compilation took place—the Corpus Juris Civilis. Justinian also oversaw the construction of the Hagia Sophia in Constantinople and the reconquest of North Africa from the Vandals and Italy from the Ostrogoths, under Belisarius (d. 565). The conquest of Italy was not complete, as a deadly outbreak of plague in 542 led to the rest of Justinian's reign concentrating on defensive measures rather than further conquests. At the emperor's death, the Byzantines had control of most of Italy, North Africa, and a small foothold in southern Spain. Justinian's reconquests have been criticised by historians for overextending his realm and setting the stage for the Muslim conquests, but many of the difficulties faced by Justinian's successors were due not just to over-taxation to pay for his wars but to the essentially civilian nature of the empire, which made raising troops difficult. [SEP] Knowledge 2: Classical music
The key characteristic of classical music that distinguishes it from popular music and folk music is that the repertoire tends to be written down in musical notation, creating a musical part or score. This score typically determines details of rhythm, pitch, and, where two or more musicians (whether singers or instrumentalists) are involved, how the various parts are coordinated. The written quality of the music has enabled a high level of complexity within them: J.S. Bach's fugues, for instance, achieve a remarkable marriage of boldly distinctive melodic lines weaving in counterpoint yet creating a coherent harmonic logic that would be impossible in the heat of live improvisation. The use of written notation also preserves a record of the works and enables Classical musicians to perform music from many centuries ago. Musical notation enables 2000s-era performers to sing a choral work from the 1300s Renaissance era or a 1700s Baroque concerto with many of the features of the music (the melodies, lyrics, forms, and rhythms) being reproduced.

Answer: John Kukuzelis