The sections in the skeleton programs where one should fill in missing
code are designated by "Begin TODO" / "End TODO" comments.
For the exercises related to heat equation, many subroutines can be
be considered as black boxes and do not require modifications.
The .pgm output files can be investigated for example with the "display" image
viewer program.
LAPACK exercises use a utility routine dlagsy for generating random symmetric
matrices with predefined eigenvalues. As all LAPACK distributions do not
provide this routine, it is included here and can be used e.g. as
gcc -o ex ex_lib_3_lapack2.c dlagsy.f (+ flags needed for LAPACK/BLAS)