Introduced Euler's phi (or totient) function. Also gave the definition of a public key cryptosystem<br>
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For more background on Euler's phi function take a look at the
wikipedia page <a href="http://en.wikipedia.org/wiki/Euler%27s_totient_function">here</a>.

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Then I introduced the idea of public key cryptography.

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See page 178 of Lawson's book for a little more on Euler's phi function. (He studied an Cambridge and he calls it Euler's totient function!)