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 A261460 Numbers k such that k^11-1 is a semiprime. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 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 LINKS EXAMPLE 20 is in sequence because 20^11-1 = 204799999999999 = 19*10778947368421, where 19 and 10778947368421 are both prime. MATHEMATICA Select[Range[6000], PrimeOmega[#^11 - 1] == 2 &] PROG (MAGMA) IsSemiprime:=func; [n: n in [2..4000] | IsSemiprime(s) where s is n^11- 1]; (PARI)  isok(n)=bigomega(n^11-1)==2 \\ Anders HellstrÃ¶m, Aug 21 2015 CROSSREFS Cf. similar sequences listed in A261435. Cf. A105122. Cf. A008864, A162862. Sequence in context: A078460 A059208 A268742 * A061471 A112271 A201122 Adjacent sequences:  A261457 A261458 A261459 * A261461 A261462 A261463 KEYWORD nonn,easy AUTHOR Vincenzo Librandi, Aug 21 2015 STATUS approved

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Last modified January 23 13:38 EST 2022. Contains 350511 sequences. (Running on oeis4.)