negative base

The use of a negative base to represent numbers gives rise to some intriguing possibilities. Consider "negadecimal," for example, in which the base is minus 10 instead of the familiar positive 10. In this system, the number 365 is equivalent to the decimal number 5 + (6 × -10) + (3 × -10 × -10), = 245, while 35 in negadecimal is equivalent to 5 + (3 × -10), = -25, in ordinary decimal. This points to an interesting fact: the negadecimal equivalent of any positive or negative decimal number is always positive and therefore doesn't need to be accompanied by a sign. The Polish UMC-1, of which a few dozen were built in the late 1950s and earlier 1960s, is the only computer ever to use "negabinary" (base 2 arithmetic).