Added the basics of Hilbert spaces.

refs.bib

@@ -196,4 +196,22 @@
   pages     = {x+207},
   isbn      = {3-540-07875-4},
   mrnumber  = {0482275}
+}
+
+@book{Yosida,
+  title={Functional analysis},
+  author={Yosida, Kösaku},
+  year={2012},
+  publisher={Springer Science \& Business Media}
+}
+
+@article{LindenstraussTzafriri1971,
+  author  = {Lindenstrauss, Joram and Tzafriri, Lior},
+  title   = {On the complemented subspaces problem},
+  journal = {Israel Journal of Mathematics},
+  volume  = {9},
+  number  = {2},
+  pages   = {263--269},
+  year    = {1971},
+  doi     = {10.1007/BF02771592}
 }