Proof of Stationary Values for Ritz Values in Lanczos Iteration

[redlegend]

I was wondering whether you have the proof to the following lemma:

"The Ritz values at step n of the Lanczos iteration are the stationary values of the Rayleigh quotient r(x)=(xT A x)/(xT x) if x is restricted to Kn "

 

[fly_ty03]

Did you ever find the proof, because I have the same exact question?