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# Planar Tur\'{a}n number of disjoint union of $C_3$ and $C_4$

Dec 2022

The {\em planar Tur\'{a}n number} of $H$, denoted by $ex_{\mathcal{P}}(n,H)$,is the maximum number of edges in an $H$-free planar graph. The planarTur\'{a}n number of $k\geq 3$ vertex-disjoint union of cycles is a trivialvalue $3n-6$. Lan, Shi and Song determine the exact value of$ex_{\mathcal{P}}(n,2C_3)$. We continue to study planar Tur\'{a}n number ofvertex-disjoint union of cycles and obtain the exact value of$ex_{\mathcal{P}}(n,H)$, where $H$ is vertex-disjoint union of $C_3$ and $C_4$.The extremal graphs are also characterized. For $k\geq 4$, we improve the lowerbound of $ex_{\mathcal{P}}(n,2C_k)$.

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