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A115393 Numbers n such that n, n-1 and n-2 are semiprimes. 1
35, 87, 95, 123, 143, 203, 215, 219, 303, 395, 447, 635, 699, 843, 923, 1043, 1139, 1263, 1347, 1403, 1643, 1763, 1839, 1895, 1943, 1983, 2103, 2183, 2219, 2307, 2363, 2435, 2463, 2519, 2643, 2723, 2735, 3099, 3387, 3603, 3695, 3867, 3903, 3959, 4287 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

a(n)=A056809(n)+2=A086005(n)+1=2*A086006(n)+1.

MAPLE

filter:= proc(n) local k;

   k:= n mod 3;

   if k = 0 then isprime(n/3) and isprime((n-1)/2) and numtheory:-bigomega(n-2)=2

   elif k= 1 then false

   else isprime((n-2)/3) and isprime((n-1)/2) and numtheory:-bigomega(n)=2

   fi

end proc:

select(filter, [seq(i, i=3..10000, 4)]); # Robert Israel, Jun 11 2020

MATHEMATICA

upto=5000; p=Prime[Range[PrimePi[upto/2]]]; lim=Floor[Sqrt[upto]]; sp={}; k=0; While[k++; p[[k]]<=lim, sp=Join[sp, p[[k]] *Take[p, {k, PrimePi[upto/p[[k]]]}]]]; sp=Sort[sp]; Transpose[Select [Partition[sp, 3, 1], Last[#]-#[[2]]==#[[2]]-First[#]==1&]][[3]] (* Harvey P. Dale, Mar 21 2011 -- semiprime generating portion of program from A001358 *)

CROSSREFS

Cf. A056809, A086005, A086006.

Sequence in context: A156126 A119691 A220007 * A086337 A118631 A220041

Adjacent sequences:  A115390 A115391 A115392 * A115394 A115395 A115396

KEYWORD

nonn

AUTHOR

Zak Seidov, Mar 08 2006

STATUS

approved

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Last modified October 23 07:27 EDT 2020. Contains 337964 sequences. (Running on oeis4.)