Observe that the fraction \frac{1}{7}\equal{}0,142857 is a pure periodical decimal with period 6\equal{}7\minus{}1,and in one period one has 142\plus{}857\equal{}999.For n\equal{}1,2,\dots find a sufficient and necessary
condition that the fraction \frac{1}{2n\plus{}1} has the same properties as above and find two such fractions other than 71. number theory unsolvednumber theory