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 A322985 Numbers k such that 123456789*10^k+1 is prime. 0
 1, 5, 17, 23, 25, 28, 91, 187, 287, 398, 899, 1364, 2921, 5125, 5890, 8780, 14881, 35689, 46669, 71861, 111710 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(22) > 1.3*10^5. All numbers up to this bound were sieved using newpgen and sr1sieve. Remaining numbers were checked for primality using Jean PennĂ©'s LLR application (BLS (N-1/N+1) test). LINKS EXAMPLE 1 is a term because 1234567891 is prime. 2 is not a term because 12345678901 is composite (it is divisible by 857). MATHEMATICA Select[Range@ 1400, PrimeQ[123456789*10^# + 1] &] (* Michael De Vlieger, Jan 04 2019 *) PROG (Python) from sympy.ntheory.primetest import isprime for n in range(1, 1000):     if isprime(123456789*10**n+1): print(n, end=', ') # Stefano Spezia, Jan 05 2019 CROSSREFS Cf. A248349, A248350, A321806. Sequence in context: A317262 A031270 A022141 * A240031 A260427 A091209 Adjacent sequences:  A322982 A322983 A322984 * A322986 A322987 A322988 KEYWORD nonn,more AUTHOR Matthias Baur, Jan 01 2019 STATUS approved

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Last modified September 20 17:16 EDT 2020. Contains 337265 sequences. (Running on oeis4.)