Any study of leadership in this dwindling third of the century must reckon with the curious figure President Reagan encounters this week on his first official venture outside the country. Prime Minister Pierre Elliott Trudeau of Canada is an enigmatic, changeable man, regarded by many as quixotic.

But amid the most trying circumstances he has held power, with only a brief interruption, for more than a dozen years. Doing business with Trudeau thus poses a true test of Reagan as president.

Many heroic traits come together in the Canadian leader. He is handsome, well born, rich, intelligent, articulate and adept at practically everything. But none of these qualities determined his political fotune. What counted more was an occasion -- the emergence of tension between Quebec, with its French culture, and the rest of the country, with its English culture, as the central issue of Canadian politics. Trudeau combined in his person the dual naitonal heritage. He rose overnight from comparative obscurity to the top of the greasy pole. Since 1968, his destiny and Canada's destiny have been intertwined.

Bilingualism, the use of French on equal terms with English throughout the country, was his first enthusiasm. It earned him enough credit in Quebec to stamp out the violent phase of the separatist movement, and then to defeat a referendum that would have put the province on the road to separatism in stages.

But Canada's identity crisis turned out to run far deeper than language. Quebec seeks a surge of economic modernization as well as more political autonomy. The other provinces have reacted against bilingualism. Those with energy riches -- Alberta and British Columbia and (more recently) the eastern provinces -- have asserted provincial rights to exploit oil and gas for their own benefit.

Against that threat of national disintegration, Trudeau has mobilized an uncompromising federalism. He is now driving through the Canadian Parliament legislation that would translate the British North American Act of 1867, which has been Canada's basic law, into a new constitution. The new constitution would protect individuals through a bill of rights but it would centralize economic power in Ottawa. It would allow for amendments by popular referendum rather than by provincial approval.

Provincial leaders, except in the Canadian heartland of Ontario, have fought back at many levels. They have opposed Trudeau in the Canadian Parliament and in the courts. They have threatened to hold back delivery of oil. As a result, the British govrnment of Margaret Thatcher has become nervous about the otherwise routine matter of handing the constitution off to Canada.

But Trudeau, fighting far his life and his country, has outflanked the provincial chauvinists by two maneuvers that emphasize Canadian nationalism. First, there is a new energy law that works to Canadianize development of oil and gas. Since the international companies own most of the drilling rights, Trudeau, instead of being locked into a petty quarrel with the provinces, is standing up for Canada against Big Oil.

Second, Trudeau has come up with a foreign policy that asserts Canada's independence from the American connection. After trying to align canada with the European Community, and then with Japan and and China in the Pacific basin, he has now embraced the Third World. He can combine with President Lopez Portillo of Mexico to squeeze the United States for more generous help to the underdeveloped countries.

The uncompromising character of the Trudeau push for central authourity has raised eyebrows all over the world. Many Canadians think that after years of fooling around, he is making a final bid to put his mark on the country. Oil companies everywhere have damned his energy policies. Not a few Americans believe that his quest for an independent foreign policy is what one high official in Washington calls "flaky."

But the United States has vital business in Canada. This country's energy future is bound up with development of Canadian resources, and the construction, across Canada, of a gas pipeline from Alaska to the nothern states. Canada and the United States share joint responsibility for continental defense. Environmental issues have to be worked out together. A fisheries treaty, important to Canada, has been held up in the Senate because of regional opposition from New England.

Establishing rapport with Trudeau, in these conditions, presents a new challenge to Reagan. The two men are almost opposite in character and experience. If only because he is fighting for national existence, Trudeau is not going to be overwhelmed by the Reagan charm. Since circumstances oblige him to move to his left, Trudeau will be turned off by conservative slogans. He can be reached and won only be something that the new president has yet to show -- mastery of a complicated subject.