2022-03-152022-03-151971https://hdl.handle.net/2152.3/11380First-day cover featuring "One plus one equals two," "Newton's Law," "Einstein's Law," "Tsiolkovsky's Law," and "Maxwell's Law" stamps.First-day cover with five stamps from The Ten Mathematical Equations that Changed the Face of the Earth series. Issued by Nicaragua on May 15, 1971.First day covers (Philately)Stamp collectingPostage-stamp albumsNicaraguaAir mail envelopes