# Person: Theodorus Of Cyrene

Theodorus was a Greek philosopher in the Cyrenaic school of moral philosophy. He was a pupil of Protagoras and the tutor of Plato and of Theaetetus.

### Mathematical Profile (Excerpt):

• Plato travelled to and from Egypt and on such occasions he spent time with Theodorus in Cyrene.
• Theodorus, however, did not spend his whole life in Cyrene for he was certainly in Athens at a time when Socrates was alive.
• A member of the society of Pythagoras, Theodorus was one of the main philosophers in the Cyrenaic school of moral philosophy.
• Our knowledge of Theodorus comes through Plato who wrote about him in his work Theaetetus.
• Our whole knowledge of Theodorus's mathematical achievements are given by this passage from Plato.
• The first point is that Plato does not credit Theodorus with a proof that the square root of two was irrational.
• This must be because √2 was proved irrational before Theodorus worked on the problem, some claim this was proved by Pythagoras himself.
• There is no doubt that Theodorus would have constructed lines of length √3, √5 etc.
• So the question which naturally comes next is how did Theodorus prove that √3, √5, ..., √17 were irrational without giving a proof which would clearly prove that any non-square number was irrational.
• It is almost impossible to conceive that Theodorus would have used this proof on each of √3, √5, ..., √17 without obtaining a general theorem long before he got to 17.
• However there is little chance to do more than guess at Theodorus's method.

Born 465 BC, Cyrene (now Shahhat, Libya). Died 398 BC, Cyrene (now Shahhat, Libya).

View full biography at MacTutor

African, Ancient Greek, Astronomy, Origin Libya, Special Numbers And Numerals

Epochs: 1

Thank you to the contributors under CC BY-SA 4.0!

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non-Github:
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### References

#### Adapted from other CC BY-SA 4.0 Sources:

1. O’Connor, John J; Robertson, Edmund F: MacTutor History of Mathematics Archive