### 8.4.6 fsolve or nSolve with the option steffenson_solver

The Steffenson method is generically the fastest method.
It combines the Newton method with a "delta-two" Aitken acceleration : with the Newton method, we obtain the sequence xi and the convergence acceleration gives the Steffenson sequence

Ri =xi −

(xi+1 − xi)2
(xi+2 − 2 xi+1 + xi)

Input :

fsolve(cos(x)=x,x,0,steffenson_solver)

Output :

0.739085133215