Friends of Boris Weisfeiler remember him as an adventurer -- a loner who often preferred his own thoughts to human companionship and at times considered people bothersome.

Five years after the 43-year-old University math professor went off on a solo hiking vacation during Penn State's Christmas break and disappeared in Chile, his colleagues remain unsure of his fate.

And the United States government continues its attempts, which have waned in recent years, to determine what became of a man many considered an algebraic genius.

"People hoped there was a chance he would show up after a while," said Richard Herman, head of the University's math department. "It's only through the passage of time that it's beginning to sink in."

State department officials are still trying to reconstruct the pieces of a puzzle that began in late December of 1984, when the Russian-born Weisfeiler set out from his Toftrees home for a hiking expedition in South America.

Published reports quoted airline records as verifying Weisfeiler's departure from New York City, and arriving in the Chilean capital of Santiago the next day. He then began his journey to a rugged area rarely visited by tourists.

When Weisfeiler failed to return for his spring semester classes, Gerard Lallement, then the math department chairman, notified authorities.

"Everybody was looking for him," Lallement said in a recent interview. "But we found no clue except his backpack, which was found with his wallet, passport and return airline tickets."

Discovered near the river where he disappeared, the items were the only trace of Weisfeiler that have surfaced.

An investigation launched by the State Department in 1985 was supplemented by a search on behalf of colleagues in the Chilean Math Society. Chile's government also assisted, but only for a time.

"The State Department case remains open," said Frances Jones, a spokeswoman for the department's Bureau of Consular Affairs in Washington, D.C.

"The investigation in Chile has stalled for a lack of new leads, but is still open," Jones said. She would not elaborate.

Officials at the U.S. Embassy in Santiago offered little more. Sonja Sweek -- the embassy's assistant press attache -- said no new evidence has surfaced.

"The Chilean government closed the case," she said. "However, the United States government is maintaining an open file so that if any new evidence should appear, we would have something to address."

Chilean officials have said they believe Weisfeiler drowned trying to cross rough waters. U.S. Embassy personnel themselves have ceased any investigations, Sweek said.

Student campaigns during the latter part of the 1980s, most prominently one by the local chapter of Amnesty International, urged Washington to step up pressure on Chile. Local legislators also began to look into the situation.

"Basically, we just intervened with the State Department," said Becky Mills, State College office manager for U.S. Rep. William F. Clinger Jr., R-Warren.

"We had been contacted by the University, and we contacted the embassy in Santiago," said Mills, whose stove is still decorated by a set of Soviet spoons, a gift from Weisfeiler for her help in obtaining his U.S. citizenship.

However, efforts have tapered off as the date of Weisfeiler's disappearance becomes more distant.

Some 10 miles from the site of Weisfeiler's disappearance lies the border to an expansive estate called Colonia Dignidad -- "Dignity Colony" -- founded by German immigrants in 1961.

Throughout the years, refugees who said they had escaped the colony accused its leader, Paul Schafer, of controlling a cult-like flock and performing bizarre rituals upon members.

Time and The Washington Post examined ties between the disappearance of Weisfeiler and the colony -- a connection Herman calls "wild speculation."

"One of the things about mathematicians is that we do speculate, but we don't put anything in print before nailing it down," he said.

The department head said reports that Weisfeiler drowned in the rapid waters of the Nuble and Los Sauces rivers fit right in with the professor's character as a self-reliant adventurer who took on terrain in Nepal, Peru, China and Alaska.

"I asked him one time why he chose to go off by himself in what to most of us are dangerous circumstances," Herman said. "He said he preferred it that way -- there was no one to bother him. He was a man of his own mind."

Between his intercontinental jaunts, Weisfeiler earned a reputation as a fine mathematician, well-versed in the field of algebraic groups -- a topic which defies simplified descriptions, Lallement and Herman say.

Born and raised in the Soviet Union, Weisfeiler graduated from Moscow State University and earned graduate degrees from the Institute of Theoretical Physics in Moscow and Leningrad's Steklous Math Institute.

He worked in Moscow until 1975, when he took a post at the Institute for Advanced Study at Princeton University.

Almost immediately after his arrival at Penn State, the National Scientific Foundation granted Weisfeiler $21,000 for a 2½-year research project in his field of specialty.

He continued research and teaching at the University until the time of his disappearance, and his absence was noticed only when he failed to show up to teach his Spring 1985 courses.

"The work he did shortly before his disappearance was viewed as his best," Herman said.

Last fall, friends of Weisfeiler in the math department launched a lecture series in his name, kicked off with lectures by Princeton math professor Armand Borel and MIT math professor Victor Kac, who both worked with Weisfeiler.

"They just came here and gave talks about his work," said Lallement. "It was in his memory."

Herman said the department plans to continue the Weisfeiler lecture series, but he has no worries that the adventurous mathematician will be forgotten by his colleagues.

"I really wish we could resolve it, but I think that's probably impossible," Herman said. "But I'm not worried about him fading from memory in the near future. He'll be in many people's thoughts."