A291611 - OEIS

A291611

Numbers k such that 5*10^k + 81 is prime.

1, 3, 6, 16, 30, 33, 36, 37, 85, 288, 561, 805, 850, 1057, 1192, 1312, 2571, 4579, 5223, 5940, 10191, 18756, 24564, 29595, 43891, 65905, 89118, 97963, 112003, 139945, 174101, 195221

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OFFSET

1,2

COMMENTS

For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section).

a(33) > 2*10^5.

LINKS

Table of n, a(n) for n=1..32.

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

Makoto Kamada, Search for 50w81.

EXAMPLE

3 is in this sequence because 5*10^3 + 81 = 5081 is prime.

Initial terms and associated primes:

a(1) = 1, 131;

a(2) = 3, 5081;

a(3) = 6, 5000081;

a(4) = 16, 50000000000000081;

a(5) = 30, 5000000000000000000000000000081; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[5*10^# + 81] &]

CROSSREFS

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

Sequence in context: A032247 A052281 A295062 * A308401 A196261 A229180

Adjacent sequences: A291608 A291609 A291610 * A291612 A291613 A291614

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Aug 27 2017

EXTENSIONS

a(29)-a(32) from Robert Price, Mar 04 2019

Terms reordered into ascending order by Robert Price, Apr 03 2022

STATUS

approved