Before and after the closure of Harper's magazine was announced last week, one potential buyer offered to keep the 130-year-old monthly alive for at least three more years.

M. J. Rossant, executive director of the Twentieth Century Fund, said the fund was mystified that its offer was turned down by Otto Silha, chairman of the Minneapolis Star and Tribune Co., Harper's parent company.

The New York-based fund asked for 10 days to two weeks to assemble an emergency meeting of its board of trustees to approve the purchase, but Silha refused, Rossant said.

"We were serious about this. We were going to give it a fair shot for a minimum of three years," Rossant said.

The fund, which sponsors research in a variety of fields, planned to establish a subsidiary to manage Harper's, which it intended to redesign, reprice and reorganize, Rossant said.

Silha could not be reached for comment today.

In announcing that Harper's would be closed after its August issue, Silha spoke of increasing losses and said: "At the same time, we were unable to reach a contractual assurance that qualified buyers would finance the magazine's long-term continuance."

Rossant said a lack of time, not lack of money, was the only problem Silha mentioned in refusing the fund's offer.

Presumably, the Star and Tribune preferred to take a tax loss estimated to be about $5 million by closing down the magazine.

Rossant said the fund's offer included no cash, but an agreement to take over Harper's unfulfilled subscriptions, which it calculated to add up to obligations of about $4 million.