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Records show that the ancient Greeks seemed unsure about the status of zero as a number: they asked themselves "How can nothing be something?", leading to interesting philosophical and, by the Medieval period, religious arguments about the nature and existence of zero and the vacuum. The paradoxes of Zeno of Elea depend in large part on the uncertain interpretation of zero. (The ancient Greeks even questioned whether 1 was a number.).... Omicron (upper case Ο, lower case ο, literally "small o": o mikron, micron meaning 'small' in contrast to omega) is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. It is not used in mathematics because it is indistinguishable from the Latin letter O. Indeed, it is not widely used because of confusion with the digit 0. Wikipedia

The ancient Greeks were not sure about the concept of zero (or of negative numbers, for that matter, although they understood fractions quite well). When they did talk about it at all, they did not use our symbol for zero. Zero came to Europe only much later, I think from India via the Arabs. By the way, the Arabs now use a dot for the number zero, and the circle that we call zero is for them the number 5.
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