An area in graph theory is graph colouring, which essentially is a labeling of the vertices or edges according to certain constraints. In this thesis we consider star edge colouring, which is a variant of proper edge colouring where we additionally require the graph to have no two-coloured paths or cycles with length 4. The smallest number of colours needed to colour a graph G with a star edge colouring is called the star chromatic index of G and is denoted <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Cchi'_%7Bst%7D(G)" />. This paper proves an upper bound of the star chromatic index of bipartite graphs in terms of the maximum degree; the maximum degree of G is the largest number of edges incident to a single vertex in G. For bipartite graphs Bk with maximum degree <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?k%5Cgeq1" />, the star chromatic index is proven to satisfy<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%20%5Cchi'_%7Bst%7D(B_k)%20%5Cleq%20k%5E2%20-%20k%20+%201" />. For bipartite graphs <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?B_%7Bk,n%7D" />, where all vertices in one part have degree n, and all vertices in the other part have degree k, it is proven that the star chromatic index satisfies <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Cchi'_%7Bst%7D(Bk,n)%20%5Cleq%20k%5E2%20-2k%20+%20n%20+%201,%20k%20%5Cgeq%20n%20%3E%201" />. We also prove an upper bound for a special case of multipartite graphs, namely <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?K_%7Bn,1,1,%5Cdots,1%7D" /> with m parts of size one. The star chromatic index of such a graph satisfies<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Cchi'_%7Bst%7D(K_%7Bn,1,1,%5Cdots,1%7D)%20%5Cleq%2015%5Clceil%5Cfrac%7Bn%7D%7B8%7D%5Crceil%5Ccdot%5Clceil%5Cfrac%7Bm%7D%7B8%7D%5Crceil%20+%20%5Cfrac%7B1%7D%7B2%7Dm(m-1),%5C,m%20%5Cgeq%205" />. For complete multipartite graphs where m < 5, we prove lower upper bounds than the one above.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:liu-164938 |
| Date | January 2020 |
| Creators | Melinder, Victor |
| Publisher | Linköpings universitet, Matematik och tillämpad matematik, Linköpings universitet, Tekniska fakulteten |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
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