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 A068814 Primes with a 2 followed by (possibly zero) 1's. 1
 2, 211, 2111, 2111111111111, 2111111111111111111, 211111111111111111111111, 2111111111111111111111111111111111111111111111111111111111, 211111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Primes of the form (19*10^n-1)/9. The next term (a(9)) has 544 digits. Harvey P. Dale, Sep 28 2015 LINKS Makoto Kamada, Prime numbers of the form 211...11. FORMULA a(n) = (19*10^A056700(n)-1)/9. MATHEMATICA Select[Table[FromDigits[Join[{2}, PadRight[{}, n, 1]]], {n, 0, 130}], PrimeQ] (* Harvey P. Dale, Sep 28 2015 *) PROG (PARI) for(n=0, 200, if(isprime(2*10^n+(10^(n)-1)/9)==1, print1(2*10^n+(10^(n)-1)/9, ", "))) CROSSREFS Cf. A056700. Sequence in context: A106100 A167839 A107612 * A170920 A215641 A195335 Adjacent sequences:  A068811 A068812 A068813 * A068815 A068816 A068817 KEYWORD nonn AUTHOR Amarnath Murthy, Mar 07 2002 EXTENSIONS More terms from Benoit Cloitre, Mar 09 2002 Edited by Ray Chandler, Feb 03 2012 STATUS approved

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Last modified April 19 01:29 EDT 2019. Contains 322237 sequences. (Running on oeis4.)