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1 (one, also called unit, unity, and (multiplicative) identity) is a number, numeral, and glyph.
In some countries, the little serif at the top is sometimes extended into a long upstroke, sometimes as long as the vertical line, which can lead to confusion with the glyph for seven in other countries.
1 is the only positive integer divisible by exactly one positive integer (whereas prime numbers are divisible by exactly two positive integers, composite numbers are divisible by more than two positive integers, and zero is divisible by all positive integers).
1 was formerly considered prime by some mathematicians, using the definition that a prime is divisible only by 1 and itself.
Unlike base 2 or base 10, this is not a positional notation.
Since the base 1 exponential function (1) always equals 1, its inverse does not exist (which would be called the logarithm base 1 if it did exist).