A290153 Numbers k such that (32*10^k - 77)/9 is prime.

2, 3, 8, 12, 15, 75, 92, 123, 126, 267, 272, 525, 561, 1041, 1947, 2571, 11667, 13656, 15981, 29852, 35256, 105530, 122733, 125751

Offset

1,1

Comments

For k>1, numbers such that the digit 3 followed by k-2 occurrences of the digit 5 followed by the digits 47 is prime (see Example section).

a(25) > 2*10^5.

Links

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 35w47.

Example

3 is in this sequence because (32*10^3 - 77)/9 = 3547 is prime.
Initial terms and primes associated:
a(1) = 2, 347;
a(2) = 3, 3547;
a(3) = 8, 355555547;
a(4) = 12; 3555555555547;
a(5) = 15, 3555555555555547; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(32*10^# - 77)/9] &]

Crossrefs

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

Sequence in context: A243192 A164817 A002958 * A266629 A249096 A249367

Adjacent sequences: A290150 A290151 A290152 * A290154 A290155 A290156

Keyword

nonn,more,hard

Author

Robert Price, Jul 21 2017

Extensions

a(22)-a(24) from Robert Price, Jun 10 2018

Status

approved