Update 'mathematics.md'

mathematics.md

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-### Euclid. *Euclid's Elements*. Translated by Thomas L. Heath, Green Lion Press, 2002.
+#### Euclid. *Euclid's Elements*. Translated by Thomas L. Heath, Green Lion Press, 2002.
 - Number Theory
     - Book VII: Definitions
     - Book VII, Proposition 34: Least common multiple of two numbers