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For an n-dimensional random field X (t) we define the excursion set A of X (t) by A = \t I₀: X (t) u\, where I₀ is the unit cube in Rⁿ. It is shown that the natural generalisation of the number of upcrossings of a one-dimensional stochastic process to random fields is via the characteristic of the set A introduced by Hadwiger (1959). This characteristic is related to the number of connected components of A. A formula is obtained for the mean value of this characteristic when n = 2, 3. This mean value is calculated explicitly when X (t) is a homogeneous Gaussian field satisfying certain regularity conditions.
Adler et al. (Sun,) studied this question.