/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/ is in libpetsc3.4.2-dev 3.4.2.dfsg1-8.1+b1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/makefile
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdm.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmcomposite.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmda.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmhide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmmesh.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmmeshdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmplex.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmplexdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmredundant.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscdmshell.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscis.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscisdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscishide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscksp.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petsckspdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscksphide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscmat.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscmatdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscmathide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscpc.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscpcdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscpchide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscsnes.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscsnesdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscsneshide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscts.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petsctsdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petsctshide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscvec.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscvecdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscvechide.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscviewer.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscviewerdef.h90
/usr/lib/petscdir/3.4.2/include/finclude/ftn-custom/petscviewerhide.h90