  The manuscript is concerned with a continuous adjoint complement to
two-dimensional, incompressible, first-order boundary-layer equations for a
flat plate boundary-layer. The text is structured into three parts. The first
part demonstrates, that the adjoint complement can be derived in two ways,
either following a first simplify then derive or a first derive and then
simplify strategy. The simplification step comprises the classical
boundary-layer (b.-l.) approximation and the derivation step transfers the
primal flow equation into a companion adjoint equation.
  The second part of the paper comprises the analyses of the coupled
primal/adjoint b.-l. framework. This leads to similarity parameters, which turn
the Partial-Differential-Equation (PDE) problem into a boundary value problem
described by a set of Ordinary-Differential-Equations (ODE) and support the
formulation of an adjoint complement to the classical Blasius equation.
Opposite to the primal Blasius equation, its adjoint complement consists of two
ODEs which can be simplified depending on the treatment of advection. It is
shown, that the advective fluxes, which are frequently debated in the
literature, vanish for the investigated self-similar b.l. flows. Differences
between the primal and the adjoint Blasius framework are discussed against
numerical solutions, and analytical expressions are derived for the adjoint
b.-l. thickness, wall shear stress and subordinated skin friction and drag
coefficients. The analysis also provides an analytical expression for the shape
sensitivity to shear driven drag objectives.
  The third part assesses the predictive agreement between the different
Blasius solutions and numerical results for Navier-Stokes simulations of a flat
plate b.-l. at Reynolds numbers between 1E+03 <= ReL <= 1E+05 .
