Published online by Cambridge University Press: 31 May 2021
We consider products of an independent and identically distributed sequence in a set  $\{f_1,\ldots ,f_m\}$ of orientation-preserving diffeomorphisms of the circle. We can naturally associate a Lyapunov exponent
$\{f_1,\ldots ,f_m\}$ of orientation-preserving diffeomorphisms of the circle. We can naturally associate a Lyapunov exponent  $\lambda $. Under few assumptions, it is known that
$\lambda $. Under few assumptions, it is known that  $\lambda \leq 0$ and that the equality holds if and only if
$\lambda \leq 0$ and that the equality holds if and only if  $f_1,\ldots ,f_m$ are simultaneously conjugated to rotations. In this paper, we state a quantitative version of this fact in the case where
$f_1,\ldots ,f_m$ are simultaneously conjugated to rotations. In this paper, we state a quantitative version of this fact in the case where  $f_1,\ldots ,f_m$ are
$f_1,\ldots ,f_m$ are  $C^k$ perturbations of rotations with rotation numbers
$C^k$ perturbations of rotations with rotation numbers  $\rho (f_1),\ldots ,\rho (f_m)$ satisfying a simultaneous diophantine condition in the sense of Moser [On commuting circle mappings and simultaneous diophantine approximations. Math. Z. 205(1) (1990), 105–121]: we give a precise estimate of
$\rho (f_1),\ldots ,\rho (f_m)$ satisfying a simultaneous diophantine condition in the sense of Moser [On commuting circle mappings and simultaneous diophantine approximations. Math. Z. 205(1) (1990), 105–121]: we give a precise estimate of  $\lambda $ (Taylor expansion) and we prove that there exist a diffeomorphism g and rotations
$\lambda $ (Taylor expansion) and we prove that there exist a diffeomorphism g and rotations  $r_i$ such that
$r_i$ such that  $\mbox {dist}(gf_ig^{-1},r_i)\ll |\lambda |^{{1}/{2}}$ for
$\mbox {dist}(gf_ig^{-1},r_i)\ll |\lambda |^{{1}/{2}}$ for  $i=1,\ldots , m$. We also state analogous results for random products of
$i=1,\ldots , m$. We also state analogous results for random products of  $2\times 2$ matrices, without any diophantine condition.
$2\times 2$ matrices, without any diophantine condition.