BULLETIN OF THE MALAYSIAN MATHEMATICAL SCIENCES SOCIETY, vol.32, no.2, pp.223-231, 2009 (Journal Indexed in SCI)
A lower triangular infinite matrix is called a triangle if there are no zeros on the principal diagonal. The main result of this paper gives a minimal set of sufficient conditions for a triangle T : A(k) -> A(k) for the sequence space A(k) defined as follows: