MLA

Berberova, N. (Nina), 1901-1993. Task, Sergei. Box 20, folder 557. 1980-91, n.d.. https://collections.library.yale.edu/catalog/16688040.

APA, 6th edition

Berberova, N. (Nina), 1901-1993. (["Box 20, folder 557"])(1980-91, n.d.)[Task, Sergei]. https://collections.library.yale.edu/catalog/16688040.