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 A108050 Integers k such that 10^k+21 is prime. 28
 1, 3, 9, 17, 55, 77, 133, 195, 357, 1537, 2629, 3409, 8007, 25671, 48003, 55811, 94983 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS There cannot be any primes of this form when k is even, because all such numbers must be divisible by 11. A number is divisible by 11 if the difference between the sum of its odd digits and the sum of its even digits is 0 or divisible by 11. When k is even, the difference is always 0. - Dmitry Kamenetsky, Jul 12 2008 The next term, if one exists, is >100000. - Robert Price, Mar 24 2011 See Kamada link - primecount.txt for terms, primesize.txt for discovery details including probable or proved primes - search on "10021". LINKS Makoto Kamada, List of near-repdigit-related prime numbers EXAMPLE For k=3 we have 10^3+21 = 1000+21 = 1021, which is prime. MATHEMATICA q=21; s=""; For[ a=q, a<=q, s="10^n+"<>ToString[ a ]<>":"; n=0; For[ i=1, i< 10^3, If[ PrimeQ[ 10^i+a ], n=1; s=s<>ToString[ i ]<>", " ]; i++ ]; If[ n>0, Print[ s ] ]; a++ ] (* Vladimir Joseph Stephan Orlovsky, May 06 2008 *) PROG (PARI) for(n=1, 1e4, if(ispseudoprime(10^n+21), print1(n", "))) \\ Charles R Greathouse IV, Jul 20 2011 CROSSREFS Cf. A049054, A088274, A088275, A138861. Sequence in context: A210337 A173140 A018307 * A343781 A321869 A009211 Adjacent sequences:  A108047 A108048 A108049 * A108051 A108052 A108053 KEYWORD more,nonn AUTHOR Julien Peter Benney (jpbenney(AT)ftml.net), Jun 01 2005 EXTENSIONS a(6)=77 inserted by Vladimir Joseph Stephan Orlovsky, May 06 2008 a(13)=8007 from Dmitry Kamenetsky, Jul 12 2008 a(14)=25671 from Robert Price, Nov 08 2010 Edited by Ray Chandler, Dec 24 2010 a(15)=48003 from Robert Price, Dec 31 2010 a(16)=55811 from Robert Price, Jan 09 2011 a(17)=94983 from Robert Price, Mar 24 2011 STATUS approved

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Last modified October 16 07:07 EDT 2021. Contains 348041 sequences. (Running on oeis4.)