In [this interview][1], Arnol'd says the following:

> My personal relation with Pontriagin
> was rather good. He invited me to his
> house and to his seminar and showed
> genuine interest in my work,
> especially on singularity theory. This
> was partially due to our common
> interests in differential topology and
> control and game theory. The main
> reason, however, was that he wanted to
> say something against me at an
> international meeting. Pontriagin was
> then the Russian representative in the
> International Mathematical Union (IMU)
> and had done a lot to prevent any vote
> for dissident Russians. (I was
> blacklisted because I, along with 99
> other mathematicians, had signed a
> letter protesting the imprisonment of
> a perfectly healthy Soviet
> mathematician in a psychiatric
> hospital. This was the standard method
> of eliminating dissidents.) The IMU
> had always been very political, and he
> succeeded.


  [1]: http://www.ams.org/notices/199704/arnold.pdf