approximation eigenvalue as initial guess

𝑥𝜆𝐴𝑥 𝑥𝑇𝑥𝜆=𝑥𝑇𝐴𝑥 𝜆=𝑥𝑇𝑘𝐴𝑥𝑘𝑥𝑇𝑘𝑥𝑘