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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 Penné's LLR application (BLS (N-1/N+1) test).

LINKS

Table of n, a(n) for n=1..21.

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.)