August 15, 1984

The International Business Machines Corporation, apparently seeking to tighten its hold on the market for sophisticated desk-top computers, today introduced an advanced version of its personal computer that experts said was so powerful it could best be compared with far more expensive systems. The new Personal Computer AT is an office computer capable of performing numerous tasks at once. It can support the work of up to three people simultaneously, with two of them at remote terminals. Because the AT uses a new microprocessor, the Intel 80286, the computer works at two to three times the speed of older I.B.M. models. But its cost, $4,000 to $6,000, depending on the configuration of the equipment, is far lower than most dealers and experts had expected.

Sales of new American-made automobiles, held down by a shortage of cars, slipped eight-tenths of 1 percent in the Aug. 1-10 period from the same period last year, when sales were unusually strong. It was the first time this year that domestic sales have declined industrywide. Analysts have been expecting a decline in sales in the second half of 1984. L. Raymond Windecker, an analyst for the Ford Motor Company, said last week, ''Considering the low inventories and the recent above- trend sales that should require some payback, a flat period would be logical.''

An article in some editions yesterday, on restrictions for reporters covering the national Democratic candidates, misstated a rule change. A prohibition on reporting or photographing the activities of candidates during campaign flights without prior permission was suspended for journalists traveling with Walter F. Mondale but retained for those covering Representative Geraldine A. Ferraro. Later developments are reported today on Page B7.

After being cheered by the multitudes in the Los Angeles Coliseum, praised by the President in a personal audience and treated to ceremonial exaltation in our nation's capital, America's Olympic medalists are to be honored by the City of New York today with the traditional tossing of tons of trash on their heads from high buildings. ''Odd custom,'' said Richard Gaines, a tourist from London visiting Wall Street yesterday. Maybe so, but New York has so honored its conquering heroes since the first ticker tape was tossed out a window in the financial district and landed on Theodore Roosevelt during a parade honoring him in 1910. ''It's the least we can do to show our appreciation,'' said Roseanna Nuciforo, an employee of E. F. Hutton, who was collecting all manner of scrap paper yesterday to throw out the window at the 221 Olympic stars expected to march in a ticker-tape parade at noon today up Broadway from Battery Park to City Hall.

LOIS SEWELL had braces put on her teeth the same day her first grandchild was born. She was 50 years old then. Nine years later Mrs. Sewell, who is president of an electrical supply company in a Boston suburb, says she doesn't regret a minute of her year-long orthodontic treatment. Roland Rattee is another of the more than 850,000 adults who wear braces. ''Before I got braces my mouth looked prehistoric,'' the 34-year-old Manhattan accountant commented. Five years after beginning the treatment he is amazed by the improvement in his appearance. Recent developments in orthodontics as well as a pervasive emphasis on beauty and health have made braces more acceptable among adults. Many of those who wear them have lived with orthodontic problems for years, but because of a lack of knowledge or money - the cost usually begins at about $2,000, with dental insurance sometimes covering a part or all - have not taken corrective measures. Some, like Mrs. Sewell, need orthodontic treatment as a prelude to bridgework or to help stop gum recession or bone loss. ''My dentist never told me'' is a frequent reaction among adults when they finally see an orthodontist.