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 A056807 Numbers n such that 3*10^n +1 is prime. 9
 1, 3, 7, 10, 28, 36, 67, 81, 147, 483, 643, 1020, 1900, 2620, 10453, 27720, 52824, 105589, 111988, 618853, 665829 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS S. W. Golomb, Properties of the sequence 3.2^n+1, Math. Comp., 30 (1976), 657-663. S. W. Golomb, Properties of the sequence 3.2^n+1, Math. Comp., 30 (1976), 657-663. [Annotated scanned copy] Makoto Kamada, Prime numbers of the form 300...001. Sabin Tabirca and Kieran Reynolds, Lacunary Prime Numbers. Primeform: User group for PFGW & PrimeForm programs. FORMULA a(n) = A101823(n) + 1. EXAMPLE n = 3 gives (3*10^3+1) = 3000+1 = 3001, which is prime. MATHEMATICA Do[ If[ PrimeQ[ 3*10^n + 1], Print[ n ]], {n, 0, 20000}] PROG (PARI) is(n)=isprime(3*10^n+1) \\ Charles R Greathouse IV, Feb 17 2017 CROSSREFS Cf. A056797, A101823, A199683, A259866. Sequence in context: A302279 A041525 A251862 * A146927 A193650 A041783 Adjacent sequences:  A056804 A056805 A056806 * A056808 A056809 A056810 KEYWORD nonn,more AUTHOR Robert G. Wilson v, Aug 22 2000 EXTENSIONS a(13) & a(14) from Julien Peter Benney (jpbenney(AT)ftml.net), Nov 23 2004 a(15) from Hugo Pfoertner a(16) & a(17) from Robert G. Wilson v, Jan 18 2005 a(18) from Roman Makarchuk, Dec 05 2008 confirmed as next term by Ray Chandler, Mar 02 2012 a(19) from Alexander Gramolin, Feb 24 2012 confirmed as next term by Ray Chandler, Mar 02 2012 a(20)-a(21) from Kamada data by Robert Price, Jan 26 2015 STATUS approved

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Last modified July 20 16:24 EDT 2019. Contains 325185 sequences. (Running on oeis4.)