What is the original paper in which Erich Hecke had first introduced the Hecke operators?



Origin of Hecke operators
Über Modulfunktionen und die Dirichletschen Reihen mit Eulerscher Produktentwicklung. I, Math. Ann. 114 (1937), 1-28; II, ibid., 316-351. These two papers are available [here](http://www.digizeitschriften.de/download/PPN235181684_0114/PPN235181684_0114___log4.pdf) and [here](http://www.digizeitschriften.de/download/PPN235181684_0114/PPN235181684_0114___log24.pdf).


