A261460 Numbers k such that k^11-1 is a semiprime.

2, 20, 30, 60, 212, 224, 258, 272, 390, 398, 480, 504, 654, 770, 812, 1040, 1194, 1448, 1698, 1748, 1874, 2000, 2238, 2274, 2294, 2438, 2522, 2664, 2714, 2790, 2802, 3020, 3138, 3168, 3300, 3392, 3434, 3794, 4160, 4232, 4518, 4722, 4968, 5334, 5654, 5658

Offset: 1

Author

Vincenzo Librandi, Aug 21 2015

Keywords

• nonn
• easy

Comments

After 2, numbers k such that k-1 and k^10 + k^9 + k^8 + k^7 + k^6 + k^5 + k^4 + k^3 + k^2 + k + 1 are both prime.

Intersection of A008864 and A162862. - Michel Marcus, Aug 21 2015

Examples

			20 is in sequence because 20^11-1 = 204799999999999 = 19*10778947368421, where 19 and 10778947368421 are both prime.
		

Crossrefs

Cf. similar sequences listed in A261435.

Cf. A105122.

Cf. A008864, A162862.

Programs

Magma

IsSemiprime:=func; [n: n in [2..4000] | IsSemiprime(s) where s is n^11- 1];

Mathematica

Select[Range[6000], PrimeOmega[#^11 - 1] == 2 &]

PARI

isok(n)=bigomega(n^11-1)==2 \\ Anders Hellström, Aug 21 2015