src/Doc/How_to_Prove_it/document/root.bib

 @string{CUP="Cambridge University Press"}
 @string{LNCS="Lect.\ Notes in Comp.\ Sci."}
 @string{Springer="Springer-Verlag"}
 
 @manual{Main,author={Tobias Nipkow},title={What's in Main},
-note={\url{http://isabelle.in.tum.de/doc/main.pdf}}}
+note={\url{https://isabelle.in.tum.de/doc/main.pdf}}}
 
 @manual{ProgProve,author={Tobias Nipkow},
 title={Programming and Proving in Isabelle/HOL},
-note={\url{http://isabelle.in.tum.de/doc/prog-prove.pdf}}}
+note={\url{https://isabelle.in.tum.de/doc/prog-prove.pdf}}}
 
 @manual{IsarRef,author={Makarius Wenzel},
 title={The Isabelle/Isar Reference Manual},
-note={\url{http://isabelle.in.tum.de/doc/isar-ref.pdf}}}
+note={\url{https://isabelle.in.tum.de/doc/isar-ref.pdf}}}