z-score rework

thesis/bib/bibliography.bib

@@ -740,6 +740,17 @@ article{ef_concept_source,
 	year = {1989},
 	month = jul,
 	pages = {205–229},
+},
+@article{zscore,
+	title = {Advanced engineering mathematics},
+	author = {Kreyszig, Erwin and Stroud, K and Stephenson, G},
+	journal = {Integration},
+	volume = {9},
+	number = {4},
+	pages = {1014},
+	year = {2008},
+	publisher = {John Wiley \& Sons, Inc. 9 th edition, 2006 Page 2 of 6 Teaching
+	             methods~…},
 }