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

This is a warning message from the linker ld indicating that it cannot find the libgfortran.so.5 library file which is required by the ../arpack/libarpack.so library file. The linker is suggesting that you try using the -rpath or -rpath-link options to specify the path to the missing library file.

Here's what you can do:

1. Check if the libgfortran.so.5 library is installed on your system. You can do this by running the following command:

   $ ldconfig -p | grep libgfortran.so.5

If the library is not found, you need to install it. On Ubuntu or Debian-based systems, you can install it using the following command:

   $ sudo apt-get install libgfortran5

2. Once you have installed the library, you need to specify its path using the -rpath or -rpath-link option when linking the libraries. For example, if the library is installed in /usr/lib/x86_64-linux-gnu/, you can use the following command to link the libraries:

   $ ld -o my_program ../arpack/libarpack.so -rpath /usr/lib/x86_64-linux-gnu/

Alternatively, you can set the LD_LIBRARY_PATH environment variable to include the path to the library file:

   $ export LD_LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/
   $ ld -o my_program ../arpack/libarpack.so

Note that this option is not recommended for production systems as it can cause conflicts with other libraries.