The Soviet Union last week informed the National Academy of Sciences (NAS) it will grant a visa to Melvyn Nathanson, honorary research fellow in Mathematics, for the current academic year, reversing an August decision to deny him entry into the USSR on the NAS Soviet-American exchange program, a spokesman for NAS said yesterday.

Nathanson, who is on leave from Southern Illinois University, where he is a professor of mathematics, said yesterday the Soviet Union presumably denied him the visa because during a 1972 stay in Moscow, Nathanson befriended a Soviet mathematician who subsequently applied to emigrate.

Nathanson said yesterday he will not accept the visa, granted for November 20, because he enjoys being at Harvard and has just signed a lease on a Cambridge apartment. However, he said he plans to ask NAS to sponsor him in a delayed trip to the Soviet Union in late spring.

The decision to grant Nathanson a visa came after repeated protests by the NAS that the Soviet Academy of Sciences and the Soviet Foreign Ministry were damaging the 18-year-old exchange program by denying the visa, Nathanson said.

"NAS was stunned--it had never happened before that a person accepted into the program was denied a visa," Nathanson said.

The U.S. Department of State raised the issue in September with the Soviet Foreign Ministry, "complaining of the hardship to the individual involved and the difficulty their denial caused the program," a spokesman for the department's Soviet relations division said yesterday.

Shlomo Sternberg, chairman of the Mathematics Department, said yesterday Harvard offered a temporary non-paying job to Nathanson while he is on a leave of absence from Southern Illinois.