  In theory and practice of inverse problems, linear operator equations $Tx=y$
with compact linear forward operators $T$ having a non-closed range
$\mathcal{R}(T)$ and mapping between infinite dimensional Hilbert spaces plays
some prominent role. As a consequence of the ill-posedness of such problems,
regularization approaches are required, and due to its unlimited qualification
spectral cut-off is an appropriate method for the stable approximate solution
of corresponding inverse problems. For this method, however, the singular
system $\{\sigma_i(T),u_i(T),v_i(T)\}_{i=1}^\infty$ of the compact operator $T$
is needed, at least for $i=1,2,...,N$, up to some stopping index $N$. In this
note we consider $n$-fold integration operators $T=J^n\;(n=1,2,...)$ in
$L^2([0,1])$ occurring in numerous applications, where the solution of the
associated operator equation is characterized by the $n$-th generalized
derivative $x=y^{(n)}$ of the Sobolev space function $y \in H^n([0,1])$. Almost
all textbooks on linear inverse problems present the whole singular system
$\{\sigma_i(J^1),u_i(J^1),v_i(J^1)\}_{i=1}^\infty$ in an explicit manner.
However, they do not discuss the singular systems for $J^n,\;n \ge 2$. We will
emphasize that this seems to be a consequence of the fact that for higher $n$
the eigenvalues $\sigma^2_i(J^n)$ of the associated ODE boundary value problems
obey transcendental equations, the complexity of which is growing with $n$. We
present the transcendental equations for $n=2,3,...$ and discuss and illustrate
the associated eigenfunctions and some of their properties.
