# Tagged Questions

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### Finding number of primes of the form $1010\ldots 101$ (in base 10) [closed]

How many primes among the positive integers, written as usual in the base $10$, are such that their digits are alternating $1$’s and $0$’s, beginning and ending with $1$?
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### do you know another Magic Square with this property?

with the repeating digits of $\frac{1}{19} = 0.052631578947368421$ we can construct an exceptional magic square : The number 19 is a cyclic number with a period of 18 before the digits start to ...
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### Period of a decimal expansion

Show that if n is a product of m distinct primes, then the period of the decimal expansion of 1/n is the lowest common multiple of the periods of 1/p over all primes p|n. I understand that the above ...
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### Length of recurrent strings of numbers in the decimal expansion of $1/p$, where $p$ is prime.

Am I right to assume that: all rational numbers have a recurrent sequence in their decimal expansion, and the length of the expansion of $1/p$, where $p$ is prime, is $p-1$ for sufficiently large ...
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### Is there a sequence of primes whose decimal representations are initial segments of each other?

I.e., is there a sequence of primes whose decimal expansions have the following form: $$a_1,\ a_1a_2,\ a_1a_2a_3,\ a_1a_2a_3a_4, \dots$$ What about with the order of the digits reversed, so each ...
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### Characterising reals with terminating decimal expansions

Show that a number has a terminating decimal expansion if and only if, it is rational and when in lowest terms, its denominator is coprime to all primes other than $2$ and $5$. This is an unsolved ...
The number $142,857$ is widely known as a cyclic number, meaning consecutive multiples are cyclic permutations, i.e. $1 × 142,857 = 142,857$ $2 × 142,857 = 285,714$ \$3 × 142,857 = ...