The simplest way to compute ℙ(Hn ≥ a) is to use the algorithm 2 in our particular case. As the number of non zero terms in R is then a2, the resulting complexity is O(n × a2). Using the proposition 4, it possible to get the same result a bit faster on very long sequence by computing the first two largest eigenvalues magnitudes λ and ν (complexity in O(a2) with Arnoldi algorithms) and to use them to compute a p-value.