† proper primen.MathematicsObsolete a prime number having the property that, when it is the denominator of a vulgar fraction, the recurring period of the equivalent decimal fraction consists of the highest possible number of figures, i.e. one less than the prime number itself.
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1864 W. H. H. Hudson in Messenger of Math. II. 1 If the period of d consist of d − 1 places, d is called a proper prime.