The distribution of the distance between a point and its nearest neighbour in a spatial Poisson process. Also the distribution of the distance from the origin in n-dimensional space to the point (X1, X2,…, Xn), where X1, X2,…, Xn are independent normal variables (see normal distribution), each with expected value 0 and variance σ2. The probability density function f is given by where σ>0 and Γ is the gamma function. The distribution has mean μ and variance V given by The distribution has mode . In the case n=2, the expressions for the mean and variance simplify to and σ2(4−π) respectively.