Added Clarke to the list of references.

refs.bib

@@ -224,3 +224,15 @@
     eprint = {https://math.stackexchange.com/q/392719},
     url = {https://math.stackexchange.com/q/392719}
 }
+
+@book{Clarke,
+  author    = {Clarke, Francis},
+  title     = {Functional Analysis, Calculus of Variations and Optimal Control},
+  series    = {Graduate Texts in Mathematics},
+  volume    = {264},
+  publisher = {Springer},
+  address   = {London},
+  year      = {2013},
+  doi       = {10.1007/978-1-4471-4820-3},
+  isbn      = {978-1-4471-4820-3}
+}