New York Attorney General Andrew M. Cuomo has started sending subpoenas and document requests this month to colleges including Columbia, Cornell, Georgetown and several State University of New York campuses as part of an investigation of relationships between the colleges and health insurance companies that cover students. The investigation appears to be focused on the adequacy of disclosure of policy terms and costs to students. Investigators also appear to be looking into whether colleges are receiving any improper payments in exchange for requiring students to use a particular insurer.