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Article Contents

ABSTRACT

1.INTRODUCTION

2.THE WARM ABSORBER

3.COLD ABSORPTION: GENERAL PROPERTIES AND WARNINGS

4.THE N_H DISTRIBUTION AMONG AGNs

5.THE LOCATION OF THE COLD X-RAY ABSORBER

6.X-RAY ABSORPTION VERSUS OPTICAL AND INFRARED ABSORPTION

7.X-RAY ABSORPTION AT HIGH LUMINOSITIES AND AT HIGH REDSHIFT

8.OPEN ISSUES

REFERENCES