80th Jubilee of Professor Nikolay Yanev

1 Early Years

Nikolay Yanev (abbr.: NY) was born on November 30, 1943 in what was then Kingdom of Bulgaria. He grew up in Pomorie, a small town on the Bulgarian Black Sea cost. His mother Smaragda was a French language teacher and his father Michail was a lawyer and legal consultant. NY spent his childhood in Pomorie, where he graduated from high school as an honor student in 1961. His favorite activities were sailing and playing trumpet in the school brass and jazz bands. At that time in Bulgaria, military service was mandatory and he was directed into the Navy. During 1961–1963 he was the helmsman of the flagman ship “Drazki” at the naval base in the city of Varna. During his military service, NY sailed more than 60,000 nautical miles. He earned a licence for helmsmen of all types of marine vessels and for captain of 100 gross register tons. During his service in the Navy, NY was also a leader of a sailor jazz-variety formation, which gave performances in both Varna and Sevastopol.

In 1964 NY was admitted as a student in the research concentration at the Faculty of Mathematics of the Sofia University “St. Kl. Ohridski”. In 1969 he completed Blocks A and B, which are equivalent to Bachelor and Master’s degree programs. He specialized in Probability Theory and Mathematical Statistics and graduated with a MS thesis. In 1969, NY started working for the section “Probability and Statistics” at the Unified Center for Mathematics and Mechanics with Computing Center at the Bulgarian Academy of Sciences (BAS).

At that time, he became familiar and fascinated by the work of Kolmogorov and his school of branching stochastic processes. In 1970 he went for a 6-month specialization at the Steklov Mathematical Institute of the Russian Academy of Sciences in Moscow. He work under the supervision of B.A. Sevastyanov. In Moscow, NY had the opportunity to attend the Kolmogorov’s seminar and witness first-hand the derivation of fundamental results in the theory of branching processes. (It is worth pointing out that back in 1940s, one of the talented Kolmogorov’s students was Nikolay Dmitriev who was of Bulgarian heritage.)

In 1972–1975 NY was a PhD student at the Faculty of Mechanics and Mathematics of the Moscow State University “M. V. Lomonosov” with academic advisor B. A. Sevastyanov. In November 1975, he successfully defended a dissertation in the field of branching stochastic processes. As a result, two seminal articles appeared in the journal “Theory of Probability and its Applications”. In the first paper [12], NY introduced and studied a class of controlled branching processes with random control functions (a.k.a. φ – branching processes with random φ). In the second paper [13], he studied the properties of statistical estimators for processes with large number of ancestors (initial particles).

2 Teaching and Advising

Upon returning to Sofia, NY rejoined the Unified Center for Mathematics and Mechanics, later renamed to Institute of Mathematics and Informatics of the BAS (IMI-BAS). He was also delivering lectures and seminars to the students of the Faculty of Mathematics and Informatics at the Sofia University (FMI-SU). NY taught the a two-semester course on Probability and Statistics for undergraduate students as well as special topics graduate level courses, including Branching Stochastic Processes, Random Walks, Renewal Theory, and Mathematical Statistics. NY is the author or co-author of the following textbooks published by Sofia University Publishing House: “Probabilities and Statistics” (co-author B. Dimitrov, 1989, 3 editions), “Manual of Exercises in Mathematical Statistics” (co-author M. Tanushev, 1989, two editions), “Branching e Stochastic Processes” (co-author M. Bojkova, 2007, two editions). These books are still being used by the students in FMI-SU.

NY has the rare gift of discovering and attracting young and talented students. The ignition of curiosity and spirit of enquiry, the transmission of knowledge and skills have the property of continuous multiplying and transforming. We, his students, appreciate all he has done for us over many years! NY was the academic advisor of eight PhD students listed below in chronological order along with their affiliations.

1. Kosto Mitov (Medical University, Pleven, Bulgaria), “Critical branching processes with state-dependent immigration” (1983),

2. Iva Tsankova (FMI-SU), “Mathematical models of the structure and organization of DNA” (1986),

3. Maroussia Bojkova (FMI-SU and IMI-BAS), “Branching migration processes” (1990),

4. George Yanev (University of Texas Rio Grande Valley, USA), “Controlled branching processes: Extinction, reproduction, migration ” (1991),

5. Vessela Stoimenova (FMI-SU), Statistical inferences of branching stochastic processes” (2005),

6. Dimitar Atanassov (New Bulgarian University, Sofia, Bulgaria), Robust methods for scaling and point estimates” (2007),

7. George Mitov (FactSet), “Randomly indexed branching processes and their application in financial mathematics” (2010),

8. Nina Daskalova (FMI-SU), “EM algorithm for estimating branching stochastic processes” (2012).

As of today, NY and his students have more than 220 publications. Three of his PhD students are now Professors, two are Associate Professors, and two are Doctors of Mathematical Sciences.

3 Research and Academic Career

In 1985 NY defended his Dr.Sci. thesis titled “Branching stochastic processes – control, statistics, applications”. The main results included there were published in The Annals of Probability [33], Journal of Applied Probability [15, 16], Advances in Applied Probability [17], Theory of Probability and its Applications [32], and Mathematical Biosciences [34]. It is interesting to mention that some of these papers were the first publications by Bulgarian authors in these journals.

NY has over 150 publications, many of them published in prestigious journals. His primary research interests are in the theory of branching stochastic processes. Let us list some of the areas of NY’s contributions along with selected publications.

1. Controlled branching processes in discrete and continuous time [12, 5],

2. Sevastyanov processes with immigration [6, 7, 21],

3. Branching processes with migration [33, 30],

4. Bellman - Harris processes with state-dependent immigration [17, 22, 18],

5. Multitype branching processes with immigration [20],

6. Renewal and regenerative processes [29, 14, 5],

7. Statistics of branching processes without and with immigration [13, 2, 3, 4].

NY worked and contributed significantly in applications of branching processes in cell biology and cancer research. He is the co-author (with A. Yakovlev) of the monograph [26]. Below we present some of NY’s areas of interest along with sample of corresponding publications.

1. Cell kinetics [26, 31, 9, 10],

2. Cell biology [23, 24, 27, 8, 25],

3. Statistics in medical science [28, 11, 1].

NY’s publications earned strong international recognition and are cited more than 500 times (including in sixteen monographs). The review [19] summarizes the research results by Bulgarian authors in the field of branching processes. More than 206 publications are included, of which 174 are by NY and his students.

NY was an invited speaker at a number of international conferences, among which we should especially mention the 1992 World Congress on Statistics of the International Statistical Institute (ISI). NY was a visiting professor in Canada (Montreal, 1990–1991), USA (Corvallis, Oregon, 1994; Rochester, New York, 2005–2009), France (Versailles, 1997–2000), Spain (Badajoz, Extremadura, 2001–2002). He collaborated with scientists from Canada, USA, Russia, France, and Spain. NY has served as a chairman and member of dissertation and academic promotion committees in Spain, France, USA, and Sweden.

Despite his excellent research results, NY was held at the inferior in rank position of a Research Fellow due to negative political references. He was finally promoted to Associate Professor in 1988. In 1991, after the fall of the communism in Bulgaria, NY was promoted to Full Professor. Two years later he was elected for Director of the IMI-BAS (1993–1999) and consequently the Head of the Probability and Statistics Section (2001–2012). Today he is a Professor Emeritus of IMI-BAS.

4 Service to the Profession

NY is the founder-father of the Bulgarian school of branching processes. In 1993, he was the Chair of the Organizing Committee of the First World Congress on Branching Processes held in the Bulgarian city of Varna. The event attracted researchers from all over the world. Under the leadership of NY, ten international conferences on probability and statistics were organized in Bulgaria.

NY served as the chairman of the Bulgarian Statistical Society. He actively participated in the activities organized by the Union of Bulgarian Mathematicians (UBM, Bulgarian Mathematical Society). For example he was one of the organizers and one of the plenary speakers of the UBM 40th Anniversary Conference.

NY has participated in the work of the General Assembly of the Bulgarian Academy of Sciences (four mandates). This was the time period of a major reorganization and modernization of the BAS. He was chairing the major Committee on “Ethics and Deformities”.

NY was actively involved in the formation and growth of the Spanish school of branching processes at the University of Extremadura. Under his leadership, strong and prolific relations were established between the University of Extremadura on the Spanish side and SU and BAS on Bulgarian side. In 2018, as recognition of his contributions, he was awarded the honorary title Doctor Honoris Causa of the University of Extremadura. The solemn inauguration ceremony took place in April 2019 in Badajoz, see Figure 2.

Figure 2

Doctor Honoris Causa, University of Extremadura, Spain.

NY is an Elected Member of the International Statistical Institute (ISI) and had presentations at two ISI World Congresses.

5 Awards and Honors

On the occasion of his 70th anniversary, NY was awarded “Marin Drinov gold medal with ribbon” , the BAS highest honorary award. In 2018 he was awarded the honorary title “Doctor Honoris Causa” of the University of Extremadura, see Figure 2. On the initiative of FMI, the Academic Council of SU “St. Kl. Ohridski” on January 29, 2020 awarded NY the honorary title Doctor Honoris Causa, see Figure 3.

Figure 3

Doctor Honoris Causa, Sofia University, Bulgaria.

NY is also an Honorary citizen of the town of Pomorie and the Chairman of the windsurfing club “Pomorie”, where he continues to be involved in sport activities. His latest passion is foil-windsurfing. Finally, it is worth noting that NY is known as Captain Nick – the best mathematician among the windsurfers and the best windsurfer among the mathematicians! In fact, he won third place at a World Windsurfing Championship for Veterans and the first two places winners were not mathematicians.

In the end, let us point out that in his 80-year life path, NY lived with dignity through three different political systems: Kingdom of Bulgaria, People’s Republic of Bulgaria (communism) and Republic of Bulgaria (democracy).

Dear Captain, Have a long life, good health, and branching success in science and sailing. Seven feet under the keel!