A159298 Numbers k such that k * 10^k * (10^k - 1)/9 + 1 is prime.

1, 5, 44, 56, 187, 192, 206, 16561

Offset

1,2

Comments

There are no more terms up to k = 4000 (cf. link).

There are no more terms up to k = 14600. - Jason Earls, Nov 30 2009

References

J. Earls, Mathematical Bliss, Pleroma Publications, 2009, page 55. ASIN: B002ACVZ6O [From Jason Earls, Nov 22 2009]

Links

Table of n, a(n) for n=1..8.

Jason Earls, Biography: Enrico Bombieri, Fields Medal winner

Example

For k=5, k * 10^k * (10^k - 1)/9 + 1 is the prime 5555500001, so 5 is a term.

Prog

(Magma) [ x[1]: x in [ < n, Integers()!(n*10^n*(10^n - 1)/9 + 1) >: n in [1..186] ] | IsPrime(x[2]) ];
(PARI) is(n)=ispseudoprime(n*10^n*(10^n-1)/9+1) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

Cf. A002275 (repunits: (10^n - 1)/9).

Sequence in context: A080284 A193458 A071861 * A262118 A173376 A364605

Adjacent sequences: A159295 A159296 A159297 * A159299 A159300 A159301

Keyword

nonn,more

Author

Lekraj Beedassy, Apr 09 2009

Extensions

Edited by Klaus Brockhaus, Apr 11 2009

a(8) from Herman Jamke (hermanjamke(AT)fastmail.fm), Oct 03 2010

Status

approved