Introduced the concepts of:
scalar multiplication for matrices,
an nxn identity matrix,
the inverse of an nxn matrix.
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Derived a formula for the inverse of a 2x2 matrix.
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Introduced the matrix affine cryptosystem.
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Yesteray I discovered that <a href="https://en.wikipedia.org/wiki/Emily_Anderson">Emily Anderson</a>, after whom the Tuesday lecture theater is named, was heavily involved in cryptography. Here is the <a href="https://www.gchq.gov.uk/information/key-figures-uk-sigint">GCHQ description of her contribution to the World War II effort</a>.