If one didn't do a double take or its cerebral equivalent upon Frank Sinatra Jr.'s sudden appearance under the lights last night at the Hyatt Regency on Capitol Hill and at the first words of "Singing the Blues" coming over the speakers, one did not come of age on this soil. Yes, the young man has an eerie resemblance to his father. That and the timbral qualities of his voice, inflections and phrasing make him almost a dead ringer for the senior Sinatra. Let it be said, however, that even with that name he'd have to have been a convincing performer or he would have been heckled off the stage by the sold-out crowd.

His program for the first show last night emphasized dramatic up-tempo numbers that contained a lot of action and many opportunities for body English, gestures, movement across the performance area and verbal asides. "The Tender Trap," "Halleluja, I Love You So" and "Night and Day," for example, were all done as veritable flag wavers, showcasing his dynamics and good feel for rhythm. Conversely, the several ballads that he included were on the sluggish side and lacked conviction.

Diversity was offered when Sinatra segued from Phil Harris' "Smoke Smoke That Cigarette" into an updated country-style comic talking blues, paid homage to the late Jack Webb with "Pete Kelly's Blues" and acknowledged Fats Waller with an interjection-laden "Until the Real Thing Comes Along."

The 18-piece Pat Valentino Concert Band (its leader and two others came with Sinatra, the rest from the area) roared, riffed and screamed its brass in the best tradition of the show band. Altogether it was a polished, upbeat and entertaining Vegas act. It plays through Sunday.