libarpack++2-dev binary package in Ubuntu Focal armhf

 ARPACK++ is an object-oriented version of the ARPACK package. It consists a
 collection of classes that offers c++ programmers an interface to ARPACK. It
 preserves the full capability, performance, accuracy and low memory
 requirements of the FORTRAN package, but takes advantage of the c++
 object-oriented programming environment.
 .
 ARPACK stands for ARnoldi PACKage and provides standard and
 generalized eigenvalue solvers for sparse matrices that arises
 typically from finite element codes. See the corresponding Debian
 package.

Source package