Here's where the common_factor headers are used: C:\Projects\boost-git\boost>dist\bin\boostdep boost/math/common_factor_ct.hpp Inclusion report for <boost/math/common_factor_ct.hpp> (in module math): from pool: <boost/pool/pool.hpp> from units: <boost/units/static_rational.hpp> from variant: <boost/variant/variant.hpp> C:\Projects\boost-git\boost>dist\bin\boostdep boost/math/common_factor_rt.hpp Inclusion report for <boost/math/common_factor_rt.hpp> (in module math): from chrono: <boost/chrono/io/duration_get.hpp> <boost/chrono/io_v1/chrono_io.hpp> from multi_index: <boost/multi_index/detail/rnd_index_node.hpp> from rational: <boost/rational.hpp> I can take care of Pool and Units, if needed. Not sure what their maintenance status is, officially.