Under special circumstances, for instance when legislation is amended, PTA may in exceptional instances alter the conditions in general authorisations and conditions for assignment of rights. The interested parties, including users, must be informed of such changes with reasonable notice, which may not, however, be less than one month. Article 13 Information on authorisations and rights PTA shall publish necessary information on rights, conditions, procedures, charges and decisions concerning general authorisations. If part of this information is held by other authorities, PTA shall publish a summary of such information and where it may be obtained. Article 14 Use of the frequency spectrum PTA shall encourage the most effective utilisation of the frequency spectrum, in accordance with international agreements thereto, and minimisation of harmful interference of reception of wireless signals. The Administration shall arrange the use of various sections of the frequency spectrum, record such arrangements, prepare a national frequency plan in accordance with this arrangement and provide information on the frequency plan as necessary. The Minister of Communications may, in a Regulation, lay down detailed rules on the arrangement and assignment of frequencies to both electronic communications undertakings and users. Article 15 Arrangement of numbers and addresses PTA shall maintain the numbering arrangements to be used for all types of electronic communications services, including number codes for networks. Information on the numbering arrangements and all changes thereto must be made public. PTA shall adopt rules1) on numbers, number series and addresses. The Minister of Communications may issue a Regulation on the role of PTA in issuing addresses. 1) Reg. 318/2003. CHAPTER V Markets Article 16 Definition of markets PTA shall define markets for goods or services and geographical markets in accordance with the principles of competition law and obligations under the Agreement on a European Economic Area (EEA). 9/34