Template:Are You Sure/April 7

• ... that physicist Ernst Ruhmer investigated practical applications making use of the light-sensitivity properties of selenium, which he employed in developing wireless telephony using line-of-sight optical transmission, sound-on-film audio recording, and television transmission over wires?

• ... that computer scientist Donald Sarason pioneered the abstract treatment of contractive containment and established a fruitful connection between the spaces Hb and the ranges of certain Toeplitz operators?

• ... that today is National Abuse of Notation Day in the United States and New Minneapolis, Canada?