  With the increasing experimental precision available at colliders,
higher-order perturbative calculations are required to reduce the theory
uncertainty in order to extract crucial QCD parameters, such as the strong
coupling constant, to the required precision, often the order of one percent.
For this reason, the contributions of the massive charm and bottom quarks need
to be taken into account in hadronic processes such as deep-inelastic
scattering. In this thesis, we calculate to order $\alpha_s^3$ the two-mass
contributions to the twist-two polarized operator matrix elements
$A_{gg,Q}^{(3)}$ and $A_{Qq}^{(3),PS}$ as functions of the heavy quark mass
ratio. We discuss the mathematical structure of these objects and discuss their
phenomenological importance for the variable flavour number scheme and
mathematical and computer-algebraic aspects of their calculation. We further
address the impact of the heavy quarks to the scheme-invariant evolution of
structure functions and introduce a Fortran library for their evaluation. We
also discuss a computer algebra implementation of a solver for partial linear
difference equations.

High Energy Physics - Phenomenology;High Energy Physics - Experiment