Iyahen attended the Saint Matthew's Primary school, Benin City (1944-45), and then the Saint Peter's School, Benin City (1945-51). These two primary schools were run by the Church Mission Society which had been founded in 1799 in the City of London, England. One of the founders was William Wilberforce who, like all the founders, was passionately committed to the abolition of the slave trade. Another founder was John Venn, the grandfather of the mathematician John Venn. The Society's mission work spread through Africa, Asia and the Middle East. The Society began work in Benin around 1900 and St Matthew's Cathedral opened in 1902. The Saint Matthew's Primary school Day School was associated with the Cathedral and run by the Church Mission Society. After one year, Iyahen moved to Saint Peter's School which had been established later, again by the Church Mission Society, and had an excellent reputation for academic record and discipline.
In 1952 Iyahen entered Edo College, Benin City, which had been established in February 1937. It is the oldest Secondary Grammar School in the Mid-Western Region. He moved to St Patrick's College, Asaba, and took the Cambridge school certificate examination, passing in Division One in 1956. He studied at Government College, Ibadan, for his Cambridge Higher School Certificate in 1957-1958 and then he worked as an Assistant Statistical Officer at the Federal Office of Statistics, Lagos in 1959 before entering University College, Ibadan later in that year. He was offered admission to study mathematics at the College, but would have loved to have studied engineering which was the preferred career of the majority of his colleagues from Edo College at that time.
University College, Ibadan, now the University of Ibadan, was established in 1948 as a College of the University of London, England; it was the first university to be established in Nigeria. It became an independent university in 1962 after the country gained its independence from Britain. At the university he was taught by Chike Obi and James Ezeilo, two of the three first Nigerian professors of mathematics. He graduated B.Sc. Hons. (Lond.) from University College, Ibadan in 1963, having been awarded First Class Honours in Mathematics. He was awarded the departmental prize for mathematics. For his postgraduate studies he went to Scotland, matriculating at the University of Glasgow in 1964.
At the University of Glasgow, Iyahen was advised by Alexander Provan Robertson (1925-1995) who had been born and brought up in Glasgow. After obtaining an M.A. at the University of Glasgow, Robertson had studied at the University of Cambridge for a Ph.D. before returning onto the staff at Glasgow. He was appointed as Professor of Mathematics at the University of Keele taking up the appointment in 1965. Previously a College, Keele had achieved university status in 1962. Ibadan moved to Keele when his advisor took up the chair of mathematics there. His pattern of working was to put in many hours during both the day and the night, and then retire to his room for siesta after lunch. His school friend William Entiemwomwan Eguagie was also undertaking research in England, in fact at Imperial College, London, and the two were guests of Mr and Mrs H A Aigbekaen in Leeds during the vacations. Iyahen was awarded a Ph.D. from the University of Keele in 1967 for his thesis On certain classes of linear topological spaces.
Iyahen's first publication was Some remarks on countably barrelled and countably quasibarrelled spaces which was published in the 1966-67 volume of the Proceedings of the Edinburgh Mathematical Society. The paper was reviewed by T Husain who wrote:-
The notions of countably barrelled and countably quasi-barrelled spaces were introduced by the reviewer (1966). These notions generalize the well known concepts of barrelled and quasi-barrelled spaces. In this paper the author gives three examples: (i) an example of a countably barrelled space which is not barrelled; (ii) an example of a countably quasi-barrelled which is not countably barrelled; (iii) an example of countably barrelled space in which a closed subspace fails to be countably barrelled.This paper gives Iyahen's address as the University of Keele but his next paper, which contained results from his thesis, On certain classes of linear topological spaces, submitted on 20 March 1967 and published in the Proceedings of the London Mathematical Society in 1968, has his address as Department of Mathematics, University of Ibadan, Nigeria. This paper has the Acknowledgement:-
The author wishes to thank Professor A P Robertson for his help and encouragement. He is grateful to I Tweddle and J W Baker for useful discussions, and to the Government of the Republic of Nigeria for financial support.Another of his papers was published in 1968, namely Semiconvex spaces in the Glasgow Mathematics Journal. He writes in the introduction to this paper:-
Many of the techniques and notions used to study various important theorems in locally convex spaces are not effective for general linear topological spaces. In 'On certain classes of linear topological spaces', a study is made of notions in general linear topological spaces which can be used to replace barrelled, bornological, and quasi-barrelled spaces. The present paper contains a parallel study in the context of semiconvex spaces.Iyahen married Veronica Aigboduwa Osagie (known as Vero) on 25 September 1967; they had five sons and one daughter, Vbenaye, Osadolor, Ewansiha, Omoruyi, Osamede and Osasumwen.
Before returning to Nigeria after the award of his Ph.D., Iyahen made a research visit to the University of Cambridge. He was then appointed as a lecturer at the University of Ibadan and, in 1969, five of his papers appeared, namely D(T, C)-spaces and the closed-graph theorem, Ultrabarrelled groups and the closed graph theorem, Subspaces of Rφ, Semiconvex spaces. II, and A note on separable locally bounded spaces.
His friend William Eguagie writes :-
It was evident that Professor Iyahen was so overtly immersed in his area of specialization that he frequently talked about Functional analysis, Topology and Vector spaces which I could hardly understand because I am not a mathematician. His books on the subject adorn my library shelves. However, I was totally impressed and overwhelmed by his achievements as a mathematician when in 1970 I went with him to University of Ife where he gave a lecture on "Infinity" at the invitation of University of Ife Mathematical Association. I was amazed when after intricate analysis of theorems; he derived and defined the concept and essence of the theory of "Infinity" only at about five minutes to the end of the one hour lecture with a standing ovation from the audience.In 1971 Iyahen was promoted to Senior Lecturer at the University of Ibadan but, one year later moved to the University of Benin where he was appointed as an Associate Professor. He was promoted to Professor at the University of Benin in 1976 and, in the same year, made Head of the Department of Mathematics. He served as Head until 1978 and then two further terms, 1980-1981 and 1988-1992. In the first break as Head of the Department of Mathematics from 1978 to 1980, he was Dean of the Faculty of Science.
Iyahen published around 30 papers between 1967 and 1989. His contributions to publishing mathematics was not restricted to research level, however, for he was also one of the authors of the six books Step by Step Mathematics for Primary Schools Books 1 (1988) Books 2 to 5 (1990). N O Omozusi writes :-
As a result of his research into the lack of enthusiasm for mathematics among youths, he discovered that such apathy arose out of lack of good foundation for the youths in mathematics even from the primary school level. This his research led to his writing six graduated books on mathematics titled "Step by Step Mathematics for Primary Schools." These books are now being used in many states of the federation. It is his fervent desire that when his effort at the primary school level would have been bearing fruit, many students would be going to the secondary school with love for mathematics and this would in turn make many students available in that field in tertiary education.He served the mathematical community in many ways in addition to his work for the University of Benin :-
He had served on the Board of West African Examinations Council (1973-1974). He was chosen to represent the University of Benin in this position. As a result of his wealth of experience, he was appointed a member of the Governing Council of the University of Maiduguri (1986-92). in order to extend the benefit of his experience to all the universities in Nigeria, the Federal government appointed him as a member of the National Universities Commission and he served in this capacity from 1993 to 1994. He served on the visitation Panel to the Rivers State university of Science and Technology, Port Harcourt in 1989. Other universities outside Nigeria also tap from his experience whether academic or administrative. Hence he had served as external examiner to several universities, among which included that of Cape Coast in Ghana, Fourah Bay University in Sierra Leone, Makerere University in Uganda, University of Nairobi in Kenya, National University in Zimbabwe and some Universities in India.He retired from the University of Benin in 1981 but served on contract as Professor of Mathematics at the University of Benin, Benin City, from 1984 to 1992. In 1983 Iyahen made the decision to enter politics. He explained why he was doing this to one of his friends, saying he wanted :-
... to avert the possibility of mediocrity creeping into the political system and our being ruled by those insensitive to the plight of the common man.In  his political work is detailed:-
Professor lyahen as a lover of his people was not satisfied with their conditions. He believed that he could improve on their condition, if he could become a policymaker. To this end he went into politics. He was elected a senator, representing the Bendel Central senatorial district, under the platform of the National party of Nigeria (N.P.N) in 1983. He so contributed his quota to the activities of that arm of legislature that he was made the chairman of senate committee on Information and Public Relations. He also served as a member of other committees, which included senate committee on Education, Committee on Science and Technology, Committee on Agriculture and Committee on rules and business. Even though the army dissolved the house of legislature within few months of its inception, Professor S O Iyahen was already being noticed both within the house of parliament and by the general Public as a man who knows the meaning of the word Toil. He and some members were working day and night in order to achieve success but the army did not give them the required chance to succeed. Here of course is no forum to examine why the army is always present in Nigerian government. His people so trusted him that he was elected again as a senator in the aborted third Republic. This time around, he was made Chairman of Senate Committee on National Planning, he was also member of senate Committee on Agriculture, Committee on Education and Committee on Water Resources. In all these capacities, he performed well. The army again struck and sacked all the legislative arms of government in Nigeria in 1993. In this second term, he was elected by the Social Democratic Party of Nigeria (S.D.P.) to represent Edo South- West senatorial District.He was appointed Coordinator of Mathematics Program at the National Mathematical Centre, Abuja, from 1998 to 2004 where he rose to become Acting Director and Chief Executive of the Centre. He was appointed by the Federal Government of Nigeria as Chairman, Governing Council, Federal Polytechnic, Idah, Kogi State 2005-2009. He became Professor of Mathematics and pioneer Head of Department of Mathematics and Computer Science at Benson Idahosa University, Benin City and Pioneer Dean of School of Postgraduate Studies of the same institution from 2010-2012.
Sunday Iyahen was awarded many honours for his outstanding mathematical contributions. He was elected a member of the African Mathematical Union (1976); he was a member of the Nigerian Mathematical Society serving as a Member of Council (1984-1988); elected a fellow of the Nigerian Academy of Science (1981) and serving as Vice President (2000-2001); and elected a fellow of the Mathematical Association of Nigeria (1992). He was Editor-in-Chief of the Afrika Mathematical the Journal of the African Mathematical Union from 1986 to 1992.
You can read a tribute by Ray O Elaho at THIS LINK.
Let us end with some short extracts from the tributes :-
From Vero [Sunday Iyahen's wife]:
Your life was about peace, love, caring and selflessness. You touched a lot of hearts. You loved and sacrificed your time and joyfully helped people unconditionally. Your selflessness, peaceful nature, heart-warming jokes and humour attracted many.From Isoken Emokpae [Sunday Iyahen's young brother]:
I didn't grow up in the same household as you, but I knew you were a brother who took the role of a father whenever you visited our late mother. Growing up, I knew you as a brother who always wanted the best for me and all of our siblings. How can I forget your laughter that emanates throughout the whole room as well as your constant humming that alerted everyone of your presence. You were the person everyone wished to have as a father, a brother, an uncle, or a friend. You were a disciplinarian, although you loved us very much. Brother Sunny, you were a man of unity and a man of peace.From Vbenaye Frank Iyahen [Sunday Iyahen's son]:
During your life time, you taught us basic core values which we cherish as a family: Integrity, Hard work, Patience, Discipline, Self-denial and Generosity.From Patrick E Iyahen [Sunday Iyahen's brother]:
My dear beloved brother Professor Senator Sunday Osarumwense Iyahen, was the embodiment of simplicity, kindness, fairness and transparency. He was always there for our family and friends, and was always ready to sacrifice his time and pleasure to make sure that others can progress. At a very tender age, he assumed the leadership and piloted the affairs of our family. With his excellent sense of direction, we were all able to accomplish our goals in life. He assumed this leadership willingly because he had a vision for us all. Not only did the immediate family benefited from his guidance and leadership, our extended family and friends also benefited. His innate desire was also carried to the political arena, were others benefited from his admirable qualities. The populace saw and believed that he was an excellent human being, transparent to the core and sincere.From Lilian Osasumwen Fatunla [Sunday Iyahen's daughter]:
I watched how excited you get when with friends or family and you erupt into your thunderous laughter, excitedly rubbing your palms together, giggling, smiling and a bubbly "Iye mwen." I watched how you constantly greeted everyone "koyo oh" just to pep up their day and make them feel better about themselves and what they do. I watched how you took on tough issues with bravery, honesty, love and wisdom even when not convenient. I observed how hard of a worker you were, how dedicated and committed you were to everything you did. With that, came resounding celebration of you from all institutions you were a part of. I watched how loving you were to my mum. You thanked her at every opportunity. Whenever my mum prepared your food, you ate with so much appreciation while repeatedly thanking her saying - "Thank you my dear, thank you my dear". I watched how honest you were with her and watched how you constantly woke up early to chat with her about your day and plans.Following his death, the Senate of the Federal Republic of Nigeria passed the following Motion :-
The Senate contends that late Senator Professor Sunday Osarumwense Iyahen was indeed a great man who served Nigeria with everything he had. He gave his all both in his youth and old age to the service of Nigeria and thus deserves to be accorded the utmost national respect even in death. Accordingly the Senate resolves to: (i) observe a minute silence in his honour; (ii) send a delegation of this hallowed chamber to pay a condolence visit on his wife, children, family members, the people and government of Edo State; and (iii) accord his burial all the official rites due such a national figure who served with diligence.
Article by: J J O'Connor and E F Robertson