Undecimal
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| Numeral systems by culture | |
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| Hindu-Arabic numerals | |
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| List of numeral system topics | |
| Positional systems by base | |
| Decimal (10) | |
| 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 20, 24, 30, 36, 60, 64 | |
| Balanced ternary | |
| Non-positional system | |
| Unary numeral system (Base 1) | |
| List of numeral systems | |
The undecimal (base-11) positional notation system is based on the number eleven, rather than ten as in decimal or eight in octal and so on. It is not a commonly used system. Undecimal requires eleven symbols representing the decimal numbers 0 through 10. For example, if the symbol for 10 were 'A', the decimal numbers 0 to 24 in undecimal would be: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 20, 21, 22. The undecimal number 1A3 would be 234 in decimal.
[edit] Base 11 in fiction
Base 11 systems appear in several science fiction stories: Carl Sagan's novel Contact references a message "hidden" inside pi that is most striking in base 11, as that permits it to be displayed in binary code. Also the fictional Psychlos (in L. Ron Hubbard's book Battlefield Earth) have a base-11 counting system.
In the television series Babylon 5, the Minbari use base-11 mathematics, according to the show's creator.
[edit] ISBN Check Digit
The check digit for ISBN is found as the result of taking modulo 11. Since this could give 11 possible results, the digit "X" is used in place of "10".
