Bäcklund devoted himself to his teaching duties with ardour and delight and he often talked about the pleasure his lectures had given him. During the time that the author (Oseen) heard him (in the late 1890s) Bäcklund carried a double burden. Outside of the regular four lectures a week he gave a beginner's course in mechanics with exercises, also four hours a week. His interest in his audience was manifested by, among other things, invitations to a yearly party where lobster was a standing ingredient.

... is a continuation of the investigations by H Hugoniot ... The paper only deals with the theories for perfect gases and makes repeated references to Helmholtz's lectures on the mathematical principles of acoustics, edited by A König and C Runge. The additions to Hugoniot's work refer to the expression for the propagation velocity of a wave when an adiabatic wave movement spreads over a slow current, and also to the propagation of a discontinuous wave in space. We also want to emphasise that an analytical derivation of the Doppler principle is given in the course of the investigation.

As is well known, there are only three different types of finite continuous irreducible contact transformation groups on the plane. In contrast, the number of different types in 3-space is considerably larger. Of these groups in space, three were first discovered by Lie and are analogous to the groups on the plane ... Scheffers also identified all groups[of a certain type] ... Engel has also set up a strange 14-membered, primitive group. Finally Kowalewski has shown that the two primitive groups mentioned are the only primitive groups of contact transformations in space. This paper contains the determination of some other classes of finite, continuous, irreducible groups of contact transformations in space. The classes examined include the two imprimitive groups set up by Lie and also four, it appears, new groups. ... With four subgroups they are similar to Lie's groups.

[Oseen], inspired by the work of the Dutch theoretical physicist H A Lorentz, posed an important problem regarding the movement of liquids. The new advance in Lorentz's research was that he took into account the viscosity of the liquid. This had usually been neglected in previous work, mainly because it considerably complicated the mathematical model. Oseen further developed Lorentz's study by considering, among other things, time-dependent fluid movements.

Dear Friend

I wish to thank you for your and your wife's visit here. You will understand what an inspiring it had on one who has not met a scientific colleague for over a year. Thanks also for both of your papers, which I read with great interest. If I pose a question concerning a-ray absorption, then please perceive this only as proof of my interest. Here is the question. Through your theory, a-ray absorption is brought into close connection with the question of ionisation by collision, for example the emission of secondary cathode rays. By your theory, how is it possible to explain hydrogen's exceptional status in this question? ... Weaker binding of the electrons?

Once again, thank you. My warmest greetings to your wife.

Your friend, C W Oseen.

Anyone familiar with modern Swedish mathematics knows the place in which it takes the problem of representing an analytic function by some unified mathematical expression in the widest possible part of its existence realm. If I had to describe the history of this problem here, I should have to remember how Mittag-Leffler devoted almost all his energy to the solution of this Weierstrassian problem. Of his successors in this field I would primarily have to recall Helge von Koch, whose extension of the Mittag-Leffler problem is of fundamental importance for what follows. However, I do not want to talk about the mathematical details here. The purpose of these lines is to show how a straightforward application of the results obtained by the colleagues mentioned should be sufficient to obtain a mathematically exact and, I believe, physically useful method of solving the problem that has for several decodes been subject to the most zealous efforts of a great number of scholars. Their actual task has been to infer the equation of state for a gas or a liquid, whose atoms interact with each other through known, but largely arbitrary forces.

Oseen's versatility was not limited to the field of physics. He successfully combined his early interest in history with his field of physics and adjacent fields through a number of biographical works. In addition to a large number of shorter life sketches of prominent Swedish scientists, including his teacher Victor Bäcklund, he wrote a larger biography of Johan Carl Wilcke. On an assignment from the Royal Swedish Academy of Sciences, he edited the edition of Scheeles' abandoned paper that was published in 1942 to celebrate the 200th anniversary of the chemist's birth. ...

Literature and art were also among Oseen's interests. He lectured on both classical and modern literature and was also a performing artist with a number of oil paintings, watercolours and drawings as a result.

Pupils and friends have spoken of Oseen's characteristic features and, in addition to his intellectual capacity and sharpness, they emphasised his willpower and work ability, which, among other things, showed itself in the laborious and complicated calculations which he quickly perceived as challenges. He asserted strongly the demand for freedom of thought and expression. The truth must be sought unconditionally. As an academic teacher, Oseen was inspiring through his engaging and interesting lectures.