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 A068817 Numbers n such that n concatenated with n 1's is a prime. 9
 1, 2, 5, 7, 10, 16, 20, 65, 91, 119, 169, 290, 428, 610, 905, 1051, 3488, 4526, 6445, 8693, 32059 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS There is no further term up to 10000. - Farideh Firoozbakht, Sep 23 2009 No more terms through 20000. - Jon E. Schoenfield, Mar 24 2018 a(22) > 50000, if it exists. - Giovanni Resta, Jun 28 2018 REFERENCES Jason Earls, On Smarandache Repunit N Numbers, Smarandache Notions Journal (2004), Vol. 14.1, pp. 251-258. LINKS EXAMPLE 5 is a member as 5 followed by five 1's, 511111, is a prime. MATHEMATICA Do[ If[ PrimeQ[ FromDigits[ Join[IntegerDigits[n], IntegerDigits[(10^n - 1)/9]]]], Print[n]], {n, 1, 1700}] PROG (PARI) for(n=1, 520, if(isprime(n*10^n+(10^n-1)/9)==1, print1(n, ", "))) CROSSREFS Cf. A070220, A070746. Sequence in context: A172410 A266594 A103871 * A156779 A345127 A226813 Adjacent sequences:  A068814 A068815 A068816 * A068818 A068819 A068820 KEYWORD nonn,base,more AUTHOR Amarnath Murthy, Mar 07 2002 EXTENSIONS More terms from Benoit Cloitre, Mar 09 2002 Further terms from Vladeta Jovovic, Amarnath Murthy and Robert G. Wilson v, May 03 2002 a(17) (a probable prime) from Rick L. Shepherd, May 10 2002 a(18)-a(19) (probable primes) from Jason Earls, Oct 15 2002 a(20) from Farideh Firoozbakht, Sep 23 2009 a(21) from Giovanni Resta, Jun 28 2018 STATUS approved

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Last modified June 13 07:19 EDT 2021. Contains 344981 sequences. (Running on oeis4.)