R. Bar-Yehuda © www.cs.technion.ac.il/~cs234141 1 קומבינטוריקה למדעי - המחשב – הרצאה #16 EULER GRAPHS גרפים אויילרים מבוסס על הספר : S. Even, "Graph Algorithms",

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R. Bar-Yehuda © 1 קומבינטוריקה למדעי - המחשב – הרצאה #16 EULER GRAPHS גרפים אויילרים מבוסס על הספר : S. Even, "Graph Algorithms", Computer Science Press, 1979 שקפים, ספר וחומר רלוונטי נוסף באתר הקורס : Slides, book and other related material at:

R. Bar-Yehuda © 2 לאונרד אוילר Leonhard Euler לאונרד אוילר Leonhard Euler 1707 נולד 1707 (באזל) 1723 מאסטר (פילוסופיה) קצין בצי הרוסי 1730 פרופסור (פיזיקה) 1733 ראש המחלקה למתימטיקה (St. Petersburg Academy of Science ) 866 ספרים ומאמרים 13 ילדים נחשב למתמטיקאי הפורה ביותר

R. Bar-Yehuda © 3 בעית טיול הגשרים בקניגסברג (Koenigsberg) בקנינגסברג היו 7 גשרים מעל נהר הפרגל (Pregel). הבעיה: כיצד ניתן לחצות את כל שבעת הגשרים מבלי לחצות גשר יותר מפעם אחת הפיתרון: ניתן לראשונה ע"י Euler ב-1736

R. Bar-Yehuda © 4 שלב א בפיתרון: ייצוג ע"י גרף

R. Bar-Yehuda © 5 EULER GRAPHS גרפים אויילרים יהי G=(V,E) גרף סופי לא מכוון. מסלול אויילרי (Euler path) ב-G הוא מסלול e 1,e 2,…, e L באורך L =|E| בו כל קשת מופיעה בדיוק פעם אחת במעגל אויילרי e 1 =e L

R. Bar-Yehuda © 6 EULER GRAPHS גרפים אויילרים גרף סופי לא-מכוון נקרא גרף אויילרי אם יש לו מסלול או מעגל אויילרי תרגיל רשות: האם הגרפים אויילרים?

R. Bar-Yehuda © EULER GRAPHS גרפים אויילרים Theorem 1.1: A finite (undirected) connected graph is an Euler graph if and only if exactly two vertices are of odd degree or all vertices are of even degree. In the latter case, every Euler path of the graph is a circuit, and in the former case, none is.

R. Bar-Yehuda © 8 גרפים מכוונים אויילרים Theorem 1.2: A finite digraph is an Euler digraph if any only if its underlying graph is connected and one of the following two conditions holds: 1.There is one vertex a s.t. d out (a) = d in (a) +1 and a vertex b s.t. d out (b)+1 = d in (b), for all other vertices v, d out (v) = d in (v). 2. For all vertices v, d out (v) = d in (v).