Equivalence or what relationship may exist between Sadurní and Serni anthroponym and, simultaneously, between the latter and their ancestors question; of how to type the first letter of the word Serni, the origin of which and without wanting to establish a new doctrine etymological can say without any vagary that the Catalan version has root or influence relationship with the other Romance languages of the environment Occitan and where it occurs in Fermí/Fermin, Serenin, Sernin, Cernin (and even so, Zadornil with respect to Castilian).
You should study the two anthtoponymsseparately, since they do not correspond to the same character but two different but related realities including: Sadurní and Serni. Let's say that this article is based on legends and, therefore, the lack of reliable documentation is clearly not conclusive helps support drawn from the study itself.
Who was Sadurní ?: The legend gives us two temporary versions: in the year 83, this disciple of the apostles preached the Christian faith in the Ribagorza and Conca de Tremp counties; yet some sources place his martyrdom by around the year 257 in Toulouse, Gaul[i].
According to legend, in the sixth century, the Leunebald Duke of Toulouse had built over his grave a church dedicated to Saint-Sernin-du-Tour (Occitan word 'bull', since he was martyred using an animal of this species).
Anyone visiting Toulouse admire the magnificent Romanesque cathedral made of bricks, without external artistic influences declared a World Heritage Site by Unesco. They say it was built in honor of the first bishop of Toulouse, Saint Saturnin (=Sant Sadurní)[ii]. The temple is an important station on the Camino de Santiago de Compostela.
Anyway, it seems that the anthroponym Sadurní is the result of Christianization and conversion of former places of worship of the pagan god Saturn-Cronos, as on this side of the Old Catalonia and elsewhere Occitan.
The Serni origin name: We can say without doubt that the current patronymic also derives from ancient words of Catalan-Occitan origin from Fermí/Fermín as Serenin, Sernin, Cernin because we can say that the phonetic and the national language character name is so derived. We know he was a disciple of Saint Saturnin.
The legend also says that Saint Fermin, patron saint of Pamplona/Iruña, the capital of Navarre, was baptized by St. Sadurní in a place of this city still known as "the Pocico of San Cernin". This name responds to an old part of the city of Pamplona said the Saint Sernin Burgo or Burgo de San Cernin where the church of San Saturnino Pamplona, also known as San Cernin[iii].
Here we find the equivalence between words that seem to refer to the same subject. Finally, we say that Aranese anthroponym is Sernin.
There are, however, who maintains that he also derived the word Zadornil force used in different family names in the border valleys of Burgos and Alava.
The regulatory and normalization of Catalan spelling now established as acceptable form: Serni.
The relationship SADURNÍ → SERNI
Greco-Roman mythology contemplated the existence of groups and couples of gods; If Castor and Pollux, for example. In these early days of Christianity there were also similarities although with different characteristics: Martha-Mary, Peter-Paul, Cosme- Damian, Cyril-Methodius, etc.
We may believe, for all that we have said, that the relationship is apparently Serni Sadurní and two different people; that is to say Sadurní is the follower of the apostles or their descendants and hierarchical Serni is a neophyte named by the first and consecrated as bishop. What tradition do not inform us properly is the existence of more than one Serni ...
The only certainty is that both saints overlap in the same story so that, today, we confuse each other. Spend in Toulouse who say the Basilica of Saint Sernin was built in honor of its first bishop Saint Saturnin; while the founding Act Anserall monastery speaks of Santum Saturninum territory and people call it San Serni, as likewise all his ancient churches and agencies that have the same surname.
Therefore, we should give and accept as true, at present, the existing equivalence San Serni = Sant Sadurní = San Saturnino.
We are aware that the same character has several anthroponyms within the same language group. James the Greater in Castilian: Santiago, Jaime, James Diego, Yago. And as the patron saint of Barcelona, Eulalia, it is assimilated to Laia which is actually a pagan goddess of the old tribe of the Lacetanian.
The correct spelling: Cerni or Serni?
Most place names and Andorran churches appear with the name written initially with "C"; It is, therefore, a customary and curial practice established since time immemorial by successive canons, bishops archivists and other students of the same orbit that have kept this particular Andorran criterion since it is a territory where their language has not been supplanted by others; as it has been able to spend the rest of Catalan-speaking countries. The Andorran legal basis that is based all this is point 1 of Article of the Law ordering the use of the official language of 1991 the government of Andorra, where it is said that the official form of place names of the Principality of Andorra is the Andorra mode, in Catalan.
This line is a very good research work conducted by Antoni Pol i Solé, architect and town planner resident in Andorra, a leading member of the Andorran Society of Sciences. Its title is The use of spelling Cerni in the Andorran tradition[iv]. Pol is a staunch defender of tradition over any adaptation or subsequent updates made by the followers of Pompeu Fabra.
On the final determination of all these and other seemingly contradictory reasoning, as mentioned Xavier Rull[v], the actions of the Commission of Toponymy of Andorra, an organization that works since 2005 and to be given, among others, also its final opinion on the case Serni serve, among other reasons, not only the tradition of the Andorran country, which is derived from the monastery of Anserall and that according to the regulations on standardization of the Catalan Institute of Catalan Studies (IEC), can considering that establishes conclusively that the phonetic form "Cerni" is written with "S". Therefore, it is clear that no single scripts for all these churches and, if necessary opt for one of the two, San Serni has more elements to his favor.
June 2015 - Josep-Maria Nogués i Torre
+Josep-Maria NOGUÉS i TORRE - ELS BENEDICTINS A TAVÈRNOLES-ANSERALL
+Antoni POL i SOLÉ - L’ÚS DE LA GRAFIA CERNI EN LA TRADICIÓ ANDORRANA.http://publicacions.iec.cat/repository/pdf/00000180%5C00000067.pdf
+Xavier RULL - L’ESTABLIMENT DE LA GRAFIA DELS TOPÒNIMS ANDORRANS: TRADICIÓ, EVOLUCIÓ I COHERÈNCIA. http://publicacions.iec.cat/repository/pdf/00000209%5C00000066.pdf
[i]Logically, there is no authentic documentof the coming holy evangelistor his presence...Undoubtedly; it was the devotion to this saint introduced in those lands by the monks of Anserall both the origins and the great epochs of its both religious expansion and economic and social domain.
[ii]The temple is an important station on the Camino de Santiago in Galicia. We high light the great condition and the beauty oft he Romanesque tower. Legend has it that fromt his stone tower that killed Simon de Montfort was launched, the head of the French army who tried unsuccessfully to take the city of Toulouse in the crusade against the Cathars or Albigensians
[iii] Information received from the cultural area of services of Pamplona City Hall.