两年前，我第一次访问组合数学中心，我很高兴有此机会能再次来访问。我去过全世界很多大学和科研机构，组合数学中心是其中规模最大、最优秀的一个研究中心之一。组合数学是数学的一个分支，在世界范围内被广泛关注，在研究组合数学的机构之中，我认为南开大学组合数学中心是占主要地位的中心之一。我不知道中国是否还有更强的组合数学中心，但是这个中心显然已经非常卓越了，这也是我为什么来这里访问的原因之一。
我非常喜欢这个中心，它做出了非常强的科研工作。这一点不仅能在你访问中心的时候了解到，还可以从中心在很多杂志上发表的论文中看出。中心产出的成果世界知名，论文都发表在国际主要的数学刊物上。你甚至不用到这里来就可以了解到中心是一个绝佳的作研究的地方。这也是我与李学良教授开始接触并开展合作的原因。
我和李学良教授远在10到15年前就开始了合作。起初，我们发现我们有一些非常相似的研究兴趣，所以我们开始互相发送邮件联系。经过一段时间之后，李学良教授安排邀请我来访问，那是大约两年前，我第一次访问南开大学。在访问期间，我们决定合作撰写一部专著，是一本很厚的书。你们可以在李学良教授那里看到这本书的样子。这本书是我在这里访问的时候开始筹划的，回国之后，我和李学良教授通过电子邮件互相交流，现在这本书已经出版。
我和李学良教授远在10到15年前就开始了合作。起初，我们发现我们有一些非常相似的研究兴趣，所以我们开始互相发送邮件联系。经过一段时间之后，李学良教授安排邀请我来访问，那是大约两年前，我第一次访问南开大学。在访问期间，我们决定合作撰写一部专著，是一本很厚的书。你们可以在李学良教授那里看到这本书的样子。这本书是我在这里访问的时候开始筹划的，回国之后，我和李学良教授通过电子邮件互相交流，现在这本书已经出版。
对于全世界的数学家而言，我们需要的科研条件几乎一样，无非是一间办公室、一块黑板、一台电脑、几张纸，还有人。其中对我们最重要的条件不是办公桌，而是可以和别人互相交流想法。这里聚集了一批高质量的数学家，这对我来说非常有趣并且非常重要。最主要的科研条件就是可以见到很多人，可以问他们问题，之后他们可以给出问题的答案。另外，这里技术上的条件和世界上其它的地方一样。数学家不需要昂贵的设备，但是我们需要思考，需要与别人交流想法。
我来自一个小国家，人口大约有一千万，仅相当于天津市。我们那里的大学规模很小，学生数量与这里相比也很少。我只有3个学生，这里的学生数量要大得多，李学良教授就有近20个学生。相比较而言，这里的学生更加优秀，同时，这里的竞争也更加激烈。因为学生们知道有很多同学都和自己一样，如果他们不去做一些工作，解决一些问题，其他人就会去做。我对中心的学生的整体看法是杰出、优秀并且努力。我们那里的学生也很杰出，很优秀，但是他们不够努力。因为他们知道如果这周没有做完工作，他们还可以下周继续做。毫无疑问，中心汇聚了中国最优秀的数学家，所以，我们对中心学生高水平的数学功底充满期待。
但是对中心的学生来说，也有一个小缺憾。中心的学生在数学上非常杰出和优秀，但是在英语方面并不突出。现在，英语是世界通用的科技语言。现在，所有的出版物都用英文出版，所有的书籍都用英文撰写，所有的会议所用的语言也是英语，而且所有像我这样的访问学者都讲英语。所以，我认为同学们应该更加努力地学习英语，因为如果英语说不好，你就无法和国外的人交流。中国很大，但是中国之外还有很多国家，与国外的学者交流也非常重要。或许，几百年之后我们会使用其它语言，但是毕竟现在世界通用的语言是英语。
我的主要研究领域是化学数学，将数学直接应用在化学中。化学数学，正如它的名字所示，涉及数学、化学两个学科。作这方面的研究，你需要了解一部分数学知识，同时还要了解一部分化学知识。事实上，熟知这两个学科的人很少，所以最好的方式是团队合作。在这个团队中，有化学方面的专家和数学方面的专家，他们可能互不理解。这时就需要像我这样的人，既懂一点化学，也懂一点数学，在团队合作中起桥梁的作用。团队合作在交叉学科中非常必要。
自然科学，不仅是数学，都是不断发展的学科，是一个知识体系。在某些时候，一些问题会以简单直接的形式出现。为了继续研究，你必须回答这些问题。在数学中也是如此。而且，在数学研究中，基础知识和问题都很重要。基础知识与实际应用并不独立。而问题往往来自于实际应用、现实生活或者自然学其它领域中的技术方面，例如人类活动等，它们可以用数学中简洁的公式表达，从中你可以看出它们与理论知识的联系。数学家的研究成果迟早会有所应用。有些数学研究有直接的应用，而有些可能只是纯理论研究。然而，数学历史证明经过一段时间之后，任何纯数学研究都会有重要的应用。研究不应该仅仅是为了应用，它也是一种纯粹的智力游戏。数学历史上有很多例子表明，虽然我们现在所作的研究可能没有即刻的实际应用，但是它的应用必定会在5年、50年或者500年之后出现。
一般而言，数学学科的发展有时会上升，有时会下降。化学图论一直保持着很好的发展趋势。我不知道它会发展到什么程度，但是在将来的5到10年间，这个学科必定会继续活跃地发展。事事无常，我不是预言家，但是我已经在这个领域从事了30年的研究，我年轻时候所做的研究工作和现在完全不同。我相信20年后它会变得更加不同，但是我确信这个学科将会一直发展下去。
这个领域中最热点的不仅仅是化学图论，还包括与理解DNA序列的涵义紧密相关的部分。DNA序列在数学的角度上看是一条含有四个符号的、承载着人类一切信息的长链。当然DNA不能完全依靠数学解码，但是数学方法无疑对研究DNA的性质非常必要，这是化学图论领域现在和将来的研究热点，当然这还需要结合物理、化学和其它相关学科。
【英文原稿】
The Most Important Research Condition—People
I have visited this center about two years ago, and now I have the pleasure to visit it again. First, I must say that I have been to many places all over the world, and I visited many universities and academic institutions. This center for combinatorics is one of the biggest places and the most outstanding places for combinatorial research. Combinatorics is a part of mathematics, which is done all over the world. There are several centers of combinatorics, I think this one in this university can be considered as one of the major centers. I don’t know if in China there is a stronger center of combinatorics, this center is certainly a very distinguished place. I must say this is one of the reasons why I come here.
After all, as what I said, I like this center. There is a very strong researching. This is not only seen when you come to the center, but you can see this when you read papers which are published in scientific publications, because the results which are produced by this center for combinatorics are known all over the world. So you don’t even need to come here to realize that this is a distinguished place for research, because the papers are published available on all major mathematical journals. Again, this is the reason how I got in touch with Professor Li and so how we started our corporations.
I started corporations with Prof Li good maybe 10, maybe more 15 years ago. We started to exchange letters because we realized that we had some very similar research interests. Then we exchange letters, and exchange letters, and finally he was able to arrange for an invitation. It’s two years ago, I came here to visit Nankai University for the first time. Then we agreed to write a joint book, so we have at the mean time finished a book, so we have a big book. Prof. Li can easily show you this book, so you can make a picture of this book. This book was initiated during my stay here, and then when I returned home, we kept exchanging further information by email. We have completed the book and it has been published in the meantime.
The conditions for research in mathematics are more or less the same everywhere in the world. You need an office, you need a blackboard, you need some papers, you need a computer and you need people. The most important thing which should be given is not only to give me a desk but also to make me can exchange ideas between these people. Here, there is a big concentration of high quality mathematicians. This is a very interesting and important place. This is the main condition. I can meet other people and ask them a question, and they can answer the question. Meeting persons is very important. The technical conditions for working here is the same as everywhere else. Mathematicians don’t need expensive equipment, but, I would say, we need something here (in the head). We have to exchange ideas.
I am coming from a small country. My whole country has about 10 million people, which is comparable with the population of Tianjin. The universities are also small. The number of students compared to what you have is small. For instance, I work with only three students, but here the number of students is much bigger. I just found that Prof Li has about 20 students. The concentration of clever mathematicians here is much higher than my country. Also, the competition here is higher, because the students know that there are many such students, if they are not to do some work, and solve some problem, then very soon somebody else will do it. So my opinion about the students is that they are outstanding, excellent and hard workers. I would say that in my country, the students are also outstanding, excellent, but they are not hard workers. Because they know that if they don’t do something in this week, they can do it in the next one. This is also well-known that there are best mathematicians collected from China. So, you should expect that the students here are very good at mathematics.
If I may say a small criticism, the students here are really excellent and outstanding in mathematics, but they are not so good in English. English today is simple a language of science. All publications are published in English, all books are written in English, all conferences use English, all visitors, like me, speak English. I think that the students should learn hard to be much better in English than they are now, because if they don’t speak English, they can’t communicate with the rest of the world. China is big, but there is also a world outside China. Sometimes it is important that you can exchange ideas with people outside. Maybe hundred of years later it will be another language, but now this is the language with which people communicate all over the world.
We are doing mathematical chemistry. We apply mathematics research directly to chemistry. As the name says, it is chemistry and mathematics, graph theory is a part of mathematics. To do this, you need some knowledge in mathematics and some knowledge in chemistry. There are very few people who have knowledge of chemistry and knowledge of mathematics. The best thing is to work together, to make a team work, in which there are some experts in chemistry who know almost nothing in mathematics and some experts in mathematics who maybe don’t know anything about chemistry. Then there must be some people, and I think I am one of them, who know a little bit of chemistry, and a little bit of mathematics. I can serve as a bridge between the chemists and the mathematicians. Team work is actually needed in such multi-discipline fields like chemical graph theory.
Science, not only mathematics, is developing. It’s a system of knowledge, and at a certain moment some questions are simply occurring. Then you have to answer these questions in order to continue to work. This is also in mathematics. Basic knowledge is not independent to applications. The questions you mentioned before is always come from applications, from the real world, from the techniques in other fields of science, in human activity, and then it is simply formulated in mathematical form which you can sometime see the relation, but the relation always comes from real life. The results which mathematician are obtained are sooner or later applied somewhere. Sometimes you do some research which are directed to some application. But sometimes you do some such pure mathematics. But history of mathematics always showed that after some times any pure mathematical research found application. Research should not intend to have any application in any place, which is just a pure intellectual game. Here I could tell you many examples from the history of mathematics that they are absolutely show that if you can’t see direct application from what we are doing now at this moment, then it will come sometime, maybe 5 years, maybe 50 years, maybe 500 years later.
In general, mathematical theories can sometimes go up, and then sometimes go down. The chemical graph theory in this moment is still going up. How far it will be, I don’t know. But, in the next 5 or 10 years you can be quite sure that the research will continue with great activity. I am not a predictor to predict what will be in the far future. Things are changing. I am now in this field of research about thirty years and I remember when I was young we did completely different things from we are doing now. It is certainly be also very different in next twenty years. But I think it will continue to exist, and this is what I am going to sure you.
The most interesting part of this field is not only chemical graph theory but also the part closely connected with the understanding of the meaning of the DNA sequence. The DNA sequence from mathematical point of view is a string of four symbols, a very long string which contains information about everything in our body. Of course, it can’t be completely decoded by mathematical methods, but mathematical approach is certainly necessary to understand its characters. I think this is the main problem which has to be treated now and in the future, of course also including the physics, chemistry and related subjects.
