/usr/bin/ld: warning: libgfortran.so.5, needed by ../arpack/libarpack.so, not found (try using -rpath or -rpath-link)

This warning message is related to the linker (ld) and indicates that the library libgfortran.so.5 is not found in the standard search directories. The library is needed by the library libarpack.so, which is being linked to your program.

To resolve this warning, you can try the following:

1. Install the missing library: You can install the libgfortran.so.5 library on your system. The method of installation may vary depending on your Linux distribution.

2. Specify the path to the library: You can use the -rpath or -rpath-link linker options to specify the path to the directory containing the libgfortran.so.5 library. For example, if the library is located in /usr/local/lib, you can use the following command to link your program:

gcc -L../arpack -Wl,-rpath-link,/usr/local/lib -o myprogram myprogram.o -larpack

This will add /usr/local/lib to the list of search directories for libraries, so that the linker can find the libgfortran.so.5 library.

3. Update LD_LIBRARY_PATH: You can also add the directory containing the libgfortran.so.5 library to the LD_LIBRARY_PATH environment variable. For example, if the library is located in /usr/local/lib, you can use the following command:

export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH

This will add /usr/local/lib to the list of directories searched by the dynamic linker when loading shared libraries.