How to type periodic numbers
When you divide 10 by 3, you get 3.33333… with an infinite number of digits after the decimal point. Such "periodic numbers" are described in the Wikipedia article about "repeating decimals".
The article also describes two common notations: dot and bar above the repeating digits. The technique is similar to that described in Letters with bars on top.
You can produce this with the Unicode characters from the "Combining Diacritical Marks" section of Unicode. The characters in this section have zero width and "overprint" the preceding character. For example, to create the digit 3 with a dot above, first type 3, then use PopChar to add the character "COMBINING DOT ABOVE". To find this character, enter "combi dot" in PopChar's search field. To add a bar, use the "COMBINING MACRON" character instead.
If you have two characters, you can also use a "COMBINING DOUBLE MACRON", which extends above the previous and next character, which avoids the gap between the two bars. To create that, first type the digits, then set the insertion point between the digits and use PopChar to insert the “COMBINING DOUBLE MACRON” character. You can also use this character to create a longer bar over multiple digits. To do so, insert it in all positions after the first and before the last digit. This creates a series of overlapping bars that appear as a single solid line. Here is a sample text that was created in TextEdit with this technique, using three examples from the Wikipedia page. As you can see, the "double macron" technique does not work equally well in all fonts. You may need to experiment to find a font in which this looks good. (*) The last version of 0.81 in these lines was made with a double macron between 8 and 1.