A285571 Numbers k such that (49*10^k + 383)/9 is prime.

1, 2, 5, 10, 19, 43, 64, 71, 127, 181, 370, 373, 742, 1085, 1171, 1438, 2038, 2269, 2819, 4802, 7742, 12010, 47120, 55129, 139442, 186409

Offset

1,2

Comments

For k>1, numbers such that the digit 5 followed by k-2 occurrences of the digit 4 followed by the digits 87 is prime (see Example section).

a(27) > 2*10^5.

Links

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Search for 54w87.

Example

4 is in this sequence because (49*10^5 + 383)/9 = 544487 is prime.
Initial terms and primes associated:
a(1) = 1, 97;
a(2) = 2, 587;
a(3) = 5, 544487;
a(4) = 10, 54444444487;
a(5) = 19, 54444444444444444487; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(49*10^# + 383)/9] &]

Crossrefs

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

Sequence in context: A304973 A016029 A018327 * A000099 A039690 A243938

Adjacent sequences: A285568 A285569 A285570 * A285572 A285573 A285574

Keyword

nonn,more,hard

Author

Robert Price, Apr 21 2017

Extensions

a(25)-a(26) from Robert Price, Apr 18 2019

Status

approved