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 A217197 Primes p such that p-3 is the greatest semiprime less than p. 1

%I

%S 13,29,61,109,137,149,181,197,229,257,277,281,317,349,389,401,457,461,

%T 541,557,569,617,677,761,797,821,929,937,977,1021,1097,1129,1217,1237,

%U 1289,1297,1321,1481,1489,1549,1597,1621,1721,1777,1861,1877,1997,2029

%N Primes p such that p-3 is the greatest semiprime less than p.

%C Subsequence of A089531.

%H T. D. Noe, <a href="/A217197/b217197.txt">Table of n, a(n) for n = 1..1000</a>

%e 977 is prime, 976 = 2^4*61 and 975 = 3*5^2*13 are not semiprimes, 974 = 2*487 is a semiprime.

%t SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; Select[Prime[Range[500]], ! SemiPrimeQ[# - 1] && ! SemiPrimeQ[# - 2] && SemiPrimeQ[# - 3] &] (* _T. D. Noe_, Sep 27 2012 *)

%o (PARI) forprime(p=5, 9999, bigomega(p-3)==2 && bigomega(p-1)!=2 && bigomega(p-2)!=2 & print1(p", "))

%Y Cf. A217195.

%K nonn

%O 1,1

%A _Antonio RoldÃ¡n_, Sep 27 2012

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Last modified January 17 20:36 EST 2020. Contains 330987 sequences. (Running on oeis4.)