Content Provider
Hathi Trust Digital Library
27
Bavarian Digital Repository (BDR - Bayerische Staatsbibliothek Munich, BSB)
13
DataCite Metadata Store (German National Library of Science and Technology)
3
Jagiellonian University: Jagiellonian Digital Library
2
University of Michigan: Digital Collections
2
Érudit.org (Université Montréal)
2
Lower Silesian Digital Library
1
NUMDAM (Archives of mathematical journals)
1
Project Gutenberg (PG)
1
Unknown
1