What was going on when I was born?

President Reagan, calling on the Soviet Union ''to reduce the tensions it has heaped on the world in the past few weeks,'' today offered what he described as concessions aimed at advancing the deadlocked Soviet-American talks on medium-range nuclear missiles in Europe. Speaking to a packed General Assembly chamber at the opening of debate in this year's session, Mr. Reagan charged that the Russians had violated arms and other agreements in the past, but declared that the United States was prepared ''to be flexible in our approach, indeed, willing to compromise,'' to win weapons agreements now. 'Door to Agreement Is Open' ''The door to an agreement is open,'' the President told the delegates, speaking of the medium-range missile talks. ''It is time for the Soviet Union to walk through it.''

With trumpets flourishing fanfares on a picture-perfect New York autumn evening, the Metropolitan Opera, America's largest, grandest performing-arts instituton, opened its centennial season last night. ''It's pretty exciting,'' said Anthony A. Bliss, the company's general manager. ''It's hard really to take in the fact this is the 100th anniversary.'' Placido Domingo, the Spanish tenor who, after some cliff-hanging indecision, went ahead and sang for the first time the difficult part of Aeneas in Berlioz's five-hour epic, ''Les Troyens,'' said: ''Opening night is always an event. All the people have come back from vacation, and they are excited to see opera again.''

An article in Business Day yesterday incorrectly identified an advertising copy writer who occasionally contributes to The National Lampoon.

Santa Fe Industries and the Southern Pacific Company, owners of two of the largest railroads serving the Western half of the nation, asked that trading in their stock be halted yesterday, amid widespread expectation that they would announce a newly negotiated merger. Spokesmen for the two companies declined comment. Al Bradshaw, a spokesman for Southern Pacific, which is based in San Francisco, said he expected the company to make an announcement today. A spokesman for Santa Fe, Robert Gehrt, said his company's board was meeting this morning in Tulsa, Okla., and any announcement would probably be made then.

A limited experimental program to control acid rain is now ''crystallizing'' at the Environmental Protection Agency and is receiving particular attention from the agency's Administrator, William D. Ruckelshaus, according to agency officials and others with whom he has discussed the issue in recent days. But even this limited approach, which would cost $1.5 billion to $2.5 billion and is one of several that have been suggested, has drawn criticism from the Energy Department and the Office of Management and Budget as premature and too costly, these officials reported, thus causing a delay in reaching a decision. No decision has been made as to whether the program would be paid for by the Federal Government, state governments, private industry or a mix. It would require substantial reductions in sulfur dioxide from power plants and factories in Ohio, Pennsylvania, West Virginia and New York, with some reductions in Vermont and New Hampshire and possibly Indiana and Michigan.

The American Express Company, resurrecting a deal that it canceled last month, announced last night that it had signed a ''definitive agreement'' to buy the Alleghany Corporation's big financial services company, Investors Diversified Services, for about $773 million in cash and stock. American Express had agreed in July to buy the company and Alleghany's small steel operations for some $115 a share but then backed out, arguing that the price was far too high. Based on Alleghany's 8.4 million shares, the new price works out to $92 a share. James D. Robinson 3d, chairman and chief executive of American Express, and Fred M. Kirby 2d, chairman and chief executive of Alleghany, who jointly announced the agreement, said Alleghany would get 11.5 million shares of American Express stock plus $337.7 million in cash. Based on yesterday's closing price for American Express stock at $37.875, the stock alone would be valued at $435.6 million.

International Brazil will receive $11 billion in loans and credits for the rest of this year and all of 1984 from the I.M.F., it was announced in Washington, where the I.M.F. and the World Bank are holding their joint annual meeting. Earlier, officials at the I.M.F. meeting gave the U.S. a victory by acting to reduce what developing nations may borrow from the fund next year. (Page A1.) At the meeting, Treasury Secretary Regan's position marks a sharp turn for the U.S. (D6.) China, in a series of new economic regulations, moved to attract greater foreign investment by giving more preferential treatment to joint ventures. The rules are expected to reduce confusion about Western investors' ties to Chinese companies. (D19.)

The latest proposals on curbing medium-range nuclear-armed missiles and planes that President Reagan outlined to the General Assembly today seem to meet some previous Soviet objections. But the speech left unresolved a fundamental difference in perception between East and West over the nuclear balance in Europe that continues to make the achieving of an accord very elusive. Basically, all the allied proposals are aimed at overcoming what the Western nations regard as a significant Soviet advantage in nuclear forces in Europe. The Soviet Union, on the other hand, denies that its forces are superior and contends that the allied proposals, if accepted, would tip the nuclear balance in the allied favor.

When the Coca-Cola Company began offering consumers a $3 rebate on a $2.99 bottle of Taylor California Cellars light wine a few months ago, it confirmed to analysts the rumor that the company's Wine Spectrum was for sale. Yesterday, Coca-Cola signed a letter of intent to sell Wine Spectrum, the country's third-largest wine producer, to Joseph E. Seagram & Sons Inc. for more than $200 million in cash. The Wine Spectrum takeover makes Seagram, the country's largest distiller, the largest publicly held wine producer in the United States as well. ''It makes them very, very big in the domestic wine business, almost too big,'' said Joe Frazzano, an analyst at Oppenheimer & Company. ''They may have to sell some.''

An article yesterday in Metropolitan Report about the search for a new chairman of the Metropolitan Transporation Authority misidentified Drew Lewis. He is chairman and chief executive officer of Warner Amex Cable Communications Inc.

Prime Minister Shafik al-Wazzan offered his resignation today to pave the way for the formation of a new Lebanese Government made up of representatives from the leading Lebanese political and religious factions. The Prime Minister tendered his resignation only a few hours after the cease-fire agreement announced Sunday night ended a month of steady fighting in and around Beirut. President Amin Gemayel immediately asked Prime Minister Wazzan and his 10-member Cabinet to stay on with full powers until proposed talks on national reconciliation begin and ''our new course becomes clear,'' the state- run National News Agency reported. Cease-Fire Largely Effective Sniping between the Lebanese army and Syrian-backed anti-Government militiamen around the front-line village of Suk al Gharb, nine miles southeast of the center of Beirut, occasionally interrupted the cease-fire that took effect at 6 A.M. today.

The Baldwin-United Corporation, the Cincinnati piano company that borrowed heavily to move into the insurance business, filed for protection under the bankruptcy laws yesterday in one of the largest financial collapses in American history. The company was a casualty of overexpansion, built on complex financial maneuvers. Its failure followed four months of intensive efforts to keep the company out of bankruptcy, and further complicated efforts to salvage the savings of thousands of Americans who bought policies from Baldwin-United subsidiaries. The filing also represented a defeat for Victor H. Palmieri, a specialist in reviving ailing corporations. Mr. Palmieri, who spent eight years rebuilding the Penn Central, took over as president of Baldwin-United in May. Baldwin-United, which has 10,000 employees nationwide and was $1.6 billion in debt, was forced to take refuge under Chapter 11 of the Federal Bankruptcy Act because of two steps by creditors. First, two banks that held $10 million of the debt refused to extend the company's loans further, Mr. Palmieri said.