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## Help with the Intel MKL interface to PARDISO

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`Dear Intel MKL experts, I would like to ask for help with the useof the Intel MKL interface to PARDISO.Assume that you have a sparse matrixA partitioned into 2x2 blocksA=    [ A_II A_IG ]        [ A_GI A_GG ],and that you want to solve linear systemswith A and A_II using only one sparse LUdecomposition of the global system A:A = [L_II        0] [U_II U_IG]      [L_GI L_GG] [0    U_GG]Neglecting pivoting for numericalstability, the solution of linear systemsof the form A_II x_I = b_I can be performedas follows:1) Solve [L_II        0] [y_I]  = [b_I]             [L_GI L_GG] [y_G] = [ * ]2) Y_G <- 03) Solve [U_II U_IG] [  x_I  ] = [y_I]             [0    U_GG] [ x_G=0 ] = [ 0 ]This algorithm can be implemented withthe Intel MKL interface of PARDISO becauseit allows to separate among forward andbackward substitution, so that you canzero-out Y_G between steps 1 and 3. The problemthat I have is that this workaroundis no longer valid when numerical pivoting enters the scene(for example, with symmetric indefinite matrices)Do you know whether there is a simple strategyto tell PARDISO to restrict numerical pivotingto a particular block (i.e., A_II) or otherwiseto tell PARDISO to solve a subblock given aglobal LU factorization of the global system ?.I want to avoid to compute both a factorizationof A_II and A.Thanks a lot for your help.Best regards,  Alberto.`