Abstract. It is proved a necessary and sufficient type theorem concerning the absolute summability of the generalized de la Vallée Poussin means of orthogonal series.

AMS Subject Classification (1991): 42C15, 40G99

Keyword(s): de la Vallée Poussion summability

Received March 13, 2012, and in revised form September 3, 2013. (Registered under 15/2012.)