ML estimate of sum of guassian variables?

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consider the sum $z=x_{1}+...+x_{k}$, where the scalar variables $x_{i}$ are statistically independent and Gaussian, each having the same mean $0$ and variance $\sigma^2_{x}.$ how can I construct the maximum likelihood estimate for the number K of the terms in the sum. How can should I consider the pdf to find maximum likelihood estimation?

Thanks in advance.