'Q.E.D.'

''Q.E.D.'' is the name of a new TV series. The letters are also the initials of the American hero of the series. Actually, Q.E.D. is short for the Latin, quod erat demonstrandum (which the show translates to mean ''quite easily done''). Not so the series, which needs more viewers to persuade CBS to extend it.

The first part of this six-part British-produced action-adventure series (Tuesdays, 8-9 p.m.) aired last week. It is unique because it combines, just a bit ambivalently, the British and American concept of series television. There's lots of vivid American action together with some witty British dialogue.

From London by transatlantic telephone, John Hawkesworth, creator and co-writer of this series, who is the man responsible for ''Upstairs, Downstairs, '' ''The Duchess of Duke Street,'' and other British classic series, is not a bit apologetic for ''Q.E.D.''

Mr. Hawkesworth is noted for using the Edwardian era for many of his series. In fact, according to him, ''Masterpiece Theater'' host Alistair Cooke once lunched with him and begged him not to do any more Edwardian series, since he was running out of Edwardian social commentary.

Isn't it a bit ironic that this new American series catches the end of the Edwardian period?

''Well, I think some of the action follows the style of the Edwardian adventure story. The good professor Q.E.D. can't stand watching people louse things up. He is arrogant, curious, and he knows he is better than other people. In addition each sequence has a valid scientific premise. I know I am making a series for America, but I want it to be a fine family show.''

Now preparing to work on a new two-hour biography of Beatrix Potter for BBC, Hawkesworth is concerned about 'Q.E.D.'s'' future, because it won't be until May that CBS gives him the decision about the go-ahead for more episodes, based on the ratings.

Meantime, he resents any implication that he has been condescending to the American market.

''No way,'' he insists.''Of course I first studied what the American market was all about. And CBS gave me some guidelines - mainly speed. But I never felt any pressure to make it more American. I did it the way I wanted to do it. Like the good professor himself, I wouldn't be able to stand seeing other people louse it up. . ."