A108240 Numbers k such that (10^k)^2 + 5*(10^k) - 1 is prime.

0, 1, 2, 7, 45, 54, 60, 99, 195, 271, 476, 1491, 2713, 8155

Offset

1,3

Comments

All of the primes for the terms <= 2713 have been certified. Primality proof for the largest: PFGW Version 20041001.Win_Stable (v1.2 RC1b) [FFT v23.8] Primality testing (10^2713)^2+5*(10^2713)-1 [N+1, Brillhart-Lehmer-Selfridge] Running N+1 test using discriminant 7, base 1+sqrt(7) Calling Brillhart-Lehmer-Selfridge with factored part 34.96% (10^2713)^2+5*(10^2713)-1 is prime! (216.4510s+10.6592s)

Links

Table of n, a(n) for n=1..14.

Prog

(PARI) is(n)=ispseudoprime((10^n)^2+5*(10^n)-1) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

Cf. A108195.

Sequence in context: A083670 A270357 A367787 * A064606 A331711 A223001

Adjacent sequences: A108237 A108238 A108239 * A108241 A108242 A108243

Keyword

more,nonn

Author

Jason Earls, Jun 17 2005

Extensions

a(14) from Michael S. Branicky, Mar 27 2023

Status

approved