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The results of this contribution are derived in the framework of functional type a posteriori error estimates. The error is measured in a combined norm which takes into account both the primal and dual variables denoted by  x  and  y , respectively. Our first main result is an error equality for all equations of the classA  *   ⁢  A  ⁢  x   +  x   =  f    ${\mathrm{A}^{*}\mathrm{A}x+x=f}$    or in mixed formulationA  *   ⁢  y   +  x   =  f    ${\mathrm{A}^{*}y+x=f}$   ,A  ⁢  x   =  y    ${\mathrm{A}x=y}$   , where the exact solution(  x  ,  y  )    $(x,y)$    is inD  ⁢   (  A  )    ×  D   ⁢   (   A  *   )$D(\mathrm{A})\times D(\mathrm{A}^{*})$   . HereA   ${\mathrm{A}}$    is a linear, densely defined and closed (usually a differential) operator andA  *    ${\mathrm{A}^{*}}$    its adjoint. In this paper we deal with very conforming mixed approximations, i.e., we assume that the approximation(   x  ~   ,   y  ~   )    ${(\tilde{x},\tilde{y})}$    belongs toD  ⁢   (  A  )    ×  D   ⁢   (   A  *   )${D(\mathrm{A})\times D(\mathrm{A}^{*})}$   . In order to obtain the exact global error value of this approximation one only needs the problem data and the mixed approximation itself, i.e., we have the  equality

Functional A Posteriori Error Control for Conforming Mixed Approximations of Coercive Problems with Lower Order Terms