A296059 Numbers k such that (10^k/8 + 1)/9 is prime.

4, 7, 16, 43, 58, 106, 160, 229, 628, 1579, 3805, 16153, 31420

Offset

1,1

Comments

Equivalently, numbers k such that (10^k + 8)/72 is prime.

a(12) > 15000. - Robert G. Wilson v, Dec 09 2017

a(14) > 10^5. - Robert Price, Apr 24 2018

Links

Table of n, a(n) for n=1..13.

Example

139, 138889, and 138888888888889 are prime, so 4, 7, and 16 are in the sequence.

Mathematica

Select[Range[2000], PrimeQ[(10^#/8 + 1)/9] &] (* Michael De Vlieger, Dec 06 2017 *)
ParallelMap[ If[ PrimeQ[(10^#/8 +1)/9], #, Nothing] &, Range@ 10000] (* Robert G. Wilson v, Dec 08 2017 *)

Prog

(PARI) isok(k) = (k>2) && isprime((10^k/8 + 1)/9); \\ Michel Marcus, Dec 05 2017

Crossrefs

Cf. A296029.

Sequence in context: A144916 A217289 A050357 * A132334 A289521 A097661

Adjacent sequences: A296056 A296057 A296058 * A296060 A296061 A296062

Keyword

nonn,more

Author

Patrick A. Thomas, Dec 04 2017

Extensions

a(8)-a(11) from Jon E. Schoenfield, Dec 04 2017

a(12)-a(13) from Robert Price, Apr 24 2018

Status

approved