  Fix $N\in\mathbb N$ and assume that for every $n\in\{1,\ldots, N\}$ the
functions $f_n\colon[0,1]\to[0,1]$ and $g_n\colon[0,1]\to\mathbb R$ are
Lebesgue measurable, $f_n$ is almost everywhere approximately differentiable
with $|g_n(x)|<|f'_n(x)|$ for almost all $x\in [0,1]$, there exists
$K\in\mathbb N$ such that the set $\{x\in [0,1]:\mathrm{card}{f_n^{-1}(x)}>K\}$
is of Lebesgue measure zero, $f_n$ satisfy Luzin's condition N, and the set
$f_n^{-1}(A)$ is of Lebesgue measure zero for every set $A\subset\mathbb R$ of
Lebesgue measure zero. We show that the formula
$Ph=\sum_{n=1}^{N}g_n\!\cdot\!(h\circ f_n)$ defines a linear and continuous
operator $P\colon L^1([0,1])\to L^1([0,1])$, and then we obtain results on the
existence and uniqueness of solutions $\varphi\in L^1([0,1])$ of the equation
$\varphi=P\varphi+g$ with a given $g\in L^1([0,1])$.
