The On-Line Encyclopedia of Integer Sequences (OEIS)

A280969 revision #15

A280969

Numbers k such that 7*10^k - 17 is prime.

1, 2, 3, 11, 16, 33, 35, 37, 39, 42, 64, 94, 123, 136, 308, 309, 310, 357, 965, 1109, 2178, 8916, 9471, 18330, 62127, 252209

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OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 83 is prime (see Example section).

a(27) > 3*10^5.

LINKS

EXAMPLE

3 is in this sequence because 7*10^3- 17 = 6983 is prime.

Initial terms and primes associated:

a(1) = 1, 53;

a(2) = 2, 683;

a(3) = 3, 6983;

a(4) = 11, 699999999983;

a(5) = 16, 69999999999999983; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[7*10^# - 17] &]

CROSSREFS

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 11 2017

EXTENSIONS

Comment corrected by Harvey P. Dale, Apr 10 2019

a(26) from Robert Price, May 31 2023

STATUS

proposed