Approximation to the Survival Functions of Quadratic Forms of Gaussian Variables

Calculates the right-tail probability of quadratic forms of Gaussian variables using the skewness-kurtosis ratio matching method, modified Liu-Tang-Zhang method and Satterthwaite-Welch method. The technical details can be found in Hong Zhang, Judong Shen and Zheyang Wu (2022) "A fast and accurate approximation to the distributions of quadratic forms of Gaussian variables" .