The motivation for this extension of topology was provided by the study of problems in stochastic processes, numerical analysis, logic, computing, control systems etc., where even though the problems considered were essentially topological the theory of topology has been found to be inadequate to handle the situation; to HA~a~I~R [9] goes the credit for this extension. Also the basic spaces of topology are effectively restricted to those with an infinity of points but the theory of extended topology will apply both to finite and infinite spaces.In 1937, CARTAZq [3,4] published two papers on filters. Bo~rRBAKI [2] used filters in his theory of convergence. SCHMIDT [10,11] and GRIMEISE~ [7,8], made further contributions to filter theory. In this paper, I will generalize the concept of filter due to CARTA~ and deal with those aspects of this generalization which will be needed to develop a theory of convergence. Definition 1. Let M denote a space and N the empty set. A family 4 of subsets of M is an extended filter or more precisely an extended filter in M iff (1) A E4 and A, BCM, A C B imply B E4. 4 is a proper extended filter iff N ¢ 4 and is the improper extended filter iff
N~4.The Cartan filter is a proper filter which satisfies the additional condition that the intersection of every two members of 4 is a member of 4. But most of the results do not depend on this condition.There are some advantages in defining an extended filter by condition (1) alone; the results in this paper can then be stated without qualifying phrases. But it turns out that for purposes of convergence it is a nuisance if N is allowed to be a member of 4-Since this paper is intended to develop results to be used in a convergence theory and since it is easy to see how the results should be modified if N E 4 I will exclude the empty set from being a member of ~. There are interesting results in the algebra of filters and they will be treated in another paper as they are not of much relevance to convergence.Hereafter filter will mean proper extended filter. Definition 2. A/a~ice is a partially ordered set in which every two-element subset (x, y} has a least upper bound x V y and a greatest lower bound x A y.A comlole~e lattice is a partially ordered set in which every non-empty subset X has a least upper bound V X and a greatest lower bound A X. I will use *