A294376 Numbers k such that 55*10^k + 7 is prime.

1, 2, 4, 7, 16, 33, 35, 51, 371, 1539, 1564, 2897, 3431, 4448, 6593, 7066, 7345, 11770, 20026, 22114, 23945, 46946, 60772

Offset

1,2

Comments

For k>0, numbers such that the digits 55 followed by k-1 occurrences of the digit 0 followed by the digit 7 is prime (see Example section).

a(24) > 3*10^5.

Links

Table of n, a(n) for n=1..23.

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

Makoto Kamada, Search for 550w7.

Example

2 is in this sequence because 55*10^2 + 7 = 5507 is prime.
Initial terms and primes associated:
a(1) = 1, 557;
a(2) = 2, 5507;
a(3) = 4, 550007;
a(4) = 7, 550000007;
a(5) = 16, 550000000000000007; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[55*10^# + 7] &]

Crossrefs

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

Sequence in context: A320465 A345333 A259544 * A259545 A319559 A260790

Adjacent sequences: A294373 A294374 A294375 * A294377 A294378 A294379

Keyword

nonn,more,hard

Author

Robert Price, Oct 29 2017

Status

approved