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A295400 Numbers k such that 3*10^k - 23 is prime. 0
1, 2, 5, 8, 14, 18, 20, 36, 68, 224, 252, 563, 780, 2430, 3150, 7919, 11092, 14020, 14908, 58032 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k > 1, numbers such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section).

a(21) > 2*10^5.

LINKS

Table of n, a(n) for n=1..20.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 29w77

EXAMPLE

2 is in this sequence because 3*10^2 - 23 = 277 is prime.

Initial terms and primes associated:

a(1) = 1, 7;

a(2) = 2, 277;

a(3) = 5, 299977;

a(4) = 8, 299999977;

a(5) = 14, 299999999999977; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[3*10^# - 23] &]

PROG

(PARI) isok(k) = isprime(3*10^k - 23); \\ Michel Marcus, Nov 22 2017

CROSSREFS

Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

Sequence in context: A173177 A191109 A190105 * A266287 A111711 A095348

Adjacent sequences:  A295397 A295398 A295399 * A295401 A295402 A295403

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 21 2017

STATUS

approved

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Last modified November 14 04:47 EST 2019. Contains 329108 sequences. (Running on oeis4.)