diff --git a/01_Topic/topic.bib b/01_Topic/topic.bib
index 0189259..fd03c4a 100644
--- a/01_Topic/topic.bib
+++ b/01_Topic/topic.bib
@@ -435,4 +435,34 @@ OCLC: 935904837}
   file = {C\:\\Users\\Hannes.Kuchelmeister\\Zotero\\storage\\NCWNAUII\\Wang und Mo - The Effect of Default Options on Consumer Decision.pdf}
 }
 
+@article{sabinProductConfigurationFrameworksa1998,
+  langid = {english},
+  title = {Product Configuration Frameworks-a Survey},
+  volume = {13},
+  issn = {1094-7167},
+  url = {http://ieeexplore.ieee.org/document/708432/},
+  doi = {10.1109/5254.708432},
+  number = {4},
+  journaltitle = {IEEE Intelligent Systems},
+  shortjournal = {IEEE Intell. Syst.},
+  urldate = {2019-10-23},
+  date = {1998-07},
+  pages = {42-49},
+  author = {Sabin, D. and Weigel, R.},
+  file = {C\:\\Users\\Hannes.Kuchelmeister\\Zotero\\storage\\BG9A29JD\\Sabin und Weigel - 1998 - Product configuration frameworks-a survey.pdf}
+}
+
+@book{tsangFoundationsConstraintSatisfaction1993,
+  langid = {english},
+  location = {{London}},
+  title = {Foundations of Constraint Satisfaction},
+  isbn = {978-0-12-701610-8},
+  pagetotal = {421},
+  series = {Computation in Cognitive Science},
+  publisher = {{Academic Press}},
+  date = {1993},
+  author = {Tsang, Edward},
+  note = {OCLC: 636781070}
+}
+
 
diff --git a/02_Proposal/proposal.bib b/02_Proposal/proposal.bib
index 0189259..fd03c4a 100644
--- a/02_Proposal/proposal.bib
+++ b/02_Proposal/proposal.bib
@@ -435,4 +435,34 @@ OCLC: 935904837}
   file = {C\:\\Users\\Hannes.Kuchelmeister\\Zotero\\storage\\NCWNAUII\\Wang und Mo - The Effect of Default Options on Consumer Decision.pdf}
 }
 
+@article{sabinProductConfigurationFrameworksa1998,
+  langid = {english},
+  title = {Product Configuration Frameworks-a Survey},
+  volume = {13},
+  issn = {1094-7167},
+  url = {http://ieeexplore.ieee.org/document/708432/},
+  doi = {10.1109/5254.708432},
+  number = {4},
+  journaltitle = {IEEE Intelligent Systems},
+  shortjournal = {IEEE Intell. Syst.},
+  urldate = {2019-10-23},
+  date = {1998-07},
+  pages = {42-49},
+  author = {Sabin, D. and Weigel, R.},
+  file = {C\:\\Users\\Hannes.Kuchelmeister\\Zotero\\storage\\BG9A29JD\\Sabin und Weigel - 1998 - Product configuration frameworks-a survey.pdf}
+}
+
+@book{tsangFoundationsConstraintSatisfaction1993,
+  langid = {english},
+  location = {{London}},
+  title = {Foundations of Constraint Satisfaction},
+  isbn = {978-0-12-701610-8},
+  pagetotal = {421},
+  series = {Computation in Cognitive Science},
+  publisher = {{Academic Press}},
+  date = {1993},
+  author = {Tsang, Edward},
+  note = {OCLC: 636781070}
+}
+