A281989 Numbers k such that 7*10^k + 61 is prime.

1, 2, 4, 6, 9, 14, 18, 20, 23, 24, 30, 61, 150, 209, 213, 226, 231, 364, 455, 3990, 15267, 24289, 28270, 94938, 155496

Offset

1,2

Comments

For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 61 is prime (see Example section).

a(27) > 3*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 70w61.

Example

4 is in this sequence because 7*10^4 + 61 = 70061 is prime.
Initial terms and associated primes:
a(1) = 1, 131;
a(2) = 2, 761;
a(3) = 4, 70061;
a(4) = 6, 7000061;
a(5) = 9, 7000000061; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[7*10^# + 61] &]

Prog

(PARI) isok(k) = isprime(7*10^k + 61); \\ Michel Marcus, Feb 04 2017

Crossrefs

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

Sequence in context: A253108 A375109 A198201 * A328423 A070211 A113753

Adjacent sequences: A281986 A281987 A281988 * A281990 A281991 A281992

Keyword

nonn,more,hard

Author

Robert Price, Feb 04 2017

Extensions

a(25) from Robert Price, Aug 20 2019

Status

approved