diff --git a/6-differentiaforms.tex b/6-differentiaforms.tex
index e7804ca..bee6a57 100644
--- a/6-differentiaforms.tex
+++ b/6-differentiaforms.tex
@@ -1,3 +1,4 @@
+\marginnote{There is a nice book by Guillemin and Haines which is all on differential forms \cite{book:guillemin-haine} and whose draft is freely available on the authors courses website.}
 In the rest of the course we will focus on a particular class of tensors, which generalizes the differential one-forms that we studied on the cotangent bundle.
 It should not be surprising then, that these will be called differential $k$-forms and that they will be alternating $(0,k)$-tensors, that is, skew-symmetric in all arguments.
 
diff --git a/aom.bbl b/aom.bbl
index 01e7c08..abfb885 100644
--- a/aom.bbl
+++ b/aom.bbl
@@ -153,10 +153,10 @@
       \field{title}{Differentiable manifolds}
       \field{year}{2014}
       \verb{urlraw}
-      \verb https://people.maths.ox.ac.uk/hitchin/files/LectureNotes/Differentiable_manifolds/manifolds2014.pdf
+      \verb http://web.archive.org/web/20201111215108/https://people.maths.ox.ac.uk/hitchin/files/LectureNotes/Differentiable_manifolds/manifolds2014.pdf
       \endverb
       \verb{url}
-      \verb https://people.maths.ox.ac.uk/hitchin/files/LectureNotes/Differentiable_manifolds/manifolds2014.pdf
+      \verb http://web.archive.org/web/20201111215108/https://people.maths.ox.ac.uk/hitchin/files/LectureNotes/Differentiable_manifolds/manifolds2014.pdf
       \endverb
     \endentry
     \entry{book:knauf}{book}{}
@@ -409,10 +409,10 @@
       \field{title}{Mathematische Physik: Klassische Mechanik}
       \field{year}{2013}
       \verb{urlraw}
-      \verb https://www.math.uni-tuebingen.de/de/forschung/maphy/personen/stefanteufel/skripte/skript2013.pdf
+      \verb http://web.archive.org/web/20201111215028/https://www.math.uni-tuebingen.de/de/forschung/maphy/personen/stefanteufel/skripte/skript2013.pdf
       \endverb
       \verb{url}
-      \verb https://www.math.uni-tuebingen.de/de/forschung/maphy/personen/stefanteufel/skripte/skript2013.pdf
+      \verb http://web.archive.org/web/20201111215028/https://www.math.uni-tuebingen.de/de/forschung/maphy/personen/stefanteufel/skripte/skript2013.pdf
       \endverb
     \endentry
     \entry{book:thirring}{book}{}
diff --git a/aom.bib b/aom.bib
index 7ab9a2d..74dfef6 100644
--- a/aom.bib
+++ b/aom.bib
@@ -39,6 +39,16 @@ @book{book:bonfigliolifulci
   url       = {https://link.springer.com/book/10.1007%2F978-3-642-22597-0}
 }
 
+@book{book:guillemin-haine,
+  doi       = {10.1142/11058},
+  url       = {http://web.archive.org/web/20200525224100/https://math.mit.edu/classes/18.952/2018SP/files/18.952_book.pdf},
+  year      = 2019,
+  month     = {mar},
+  publisher = {{WORLD} {SCIENTIFIC}},
+  author    = {Victor Guillemin and Peter Haine},
+  title     = {Differential Forms}
+}
+
 @book{book:knauf,
   title     = {Mathematical physics: classical mechanics},
   author    = {Knauf, Andreas},
@@ -130,7 +140,7 @@ @misc{lectures:hitchin
   title  = {Differentiable manifolds},
   year   = {2014},
   note   = {Unpublished lecture notes},
-  url    = {https://people.maths.ox.ac.uk/hitchin/files/LectureNotes/Differentiable_manifolds/manifolds2014.pdf}
+  url    = {http://web.archive.org/web/20201111215108/https://people.maths.ox.ac.uk/hitchin/files/LectureNotes/Differentiable_manifolds/manifolds2014.pdf}
 }
 
 @misc{lectures:merry,
@@ -146,5 +156,5 @@ @misc{lectures:teufel
   title  = {Mathematische Physik: Klassische Mechanik},
   year   = {2013},
   note   = {Unpublished lecture notes},
-  url    = {https://www.math.uni-tuebingen.de/de/forschung/maphy/personen/stefanteufel/skripte/skript2013.pdf}
+  url    = {http://web.archive.org/web/20201111215028/https://www.math.uni-tuebingen.de/de/forschung/maphy/personen/stefanteufel/skripte/skript2013.pdf}
 }