2、3n-6

3、2n-4

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If the degree of each surface of a plane graph is at least l(l≥2), there is m≤l/(l-2)×(n-2), which is an inference of Euler's formula. The number of times on each side of the second sheet is at least 3 times, and the number of times on each side of the third sheet is at least 4 times.