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

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

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 A347140

Adjacent sequences: A115390 A115391 A115392 * A115394 A115395 A115396

Keyword

nonn

Author

Zak Seidov, Mar 08 2006

Status

approved