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A115393 Numbers n such that n, n-1 and n-2 are semiprimes. 0
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

Table of n, a(n) for n=1..45.

FORMULA

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

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 March 27 13:47 EDT 2017. Contains 284176 sequences.