N.Y.C. Charter § 1121: Section 1121. Agreements concerning performance of agency administrative functions.

Notwithstanding any other provision of local law to the contrary, two or more agencies of the city may, by mutual agreement, share in the performance of specified administrative functions or designate one or more of such agencies to perform such functions for one or more other such agencies if, in the judgment of the heads of such agencies, such sharing or designation will result in more effective or efficient performance of such functions for the agencies entering into such agreement. An agreement pursuant to this section may include but shall not be limited to the sharing of, or designation of one or more agencies to perform, one or more of the following functions: personnel services, labor relations, facilities maintenance and management, purchasing, information technology and telecommunications, budget administration, and internal auditing. For purposes of this section, "agencies of the city" shall include but not be limited to mayoral and non-mayoral agencies, city boards and commissions, and the offices of elected city officers.