The *Seifert-van Kampen theorem* is the classical theorem of algebraic topology that the fundamental group functor $\pi\_1$ preserves pushouts; more often than not this is referred to simply as the *van Kampen theorem*, with no *Seifert* attached. Curious as to why, I tried looking up the history of the theorem, and (in the few sources at my immediate disposal) could only find mention of van Kampen; the wikipedia page of Herbert Seifert doesn't even mention the theorem.


So my question is: **Why does the theorem bear Seifert's name?**



What was Seifert's contribution to the Seifert-van Kampen theorem?
According to Jahrbuch der Mathematik this theorem is in:


* Seifert, H.
Konstruktion dreidimensionaler geschlossener Räume. (German) JFM 57.0723.01
Berichte Leipzig 83, 26-66. Technische Hochschule Dresden, Diss (1931).


See the (long) 
[review](http://zbmath.org/?q=an:57.0723.01) in zbMATH by Erika Pannwitz. The relevant sentence is:


* (Die erforderlichen Hilfsmittel werden in § 2 und 3 der Arbeit zusammengestellt; § 3 enthält insbesondere Aussagen über die Fundamentalgruppe eines Komplexes, der aus zwei Komplexen mit zusammenhängendem Durchschnitt zusammengesetzt ist.)


See also footnote 65 on page 19 of:


* [Klaus Volkert: Die Beiträge von Seifert und Threlfall zur dreidimensionalen Topologie.](http://www2.math.uni-wuppertal.de/~volkert/SeifertThrelfall.pdf)


