A256381 Numbers n such that n-3 and n+3 are semiprimes.

7, 12, 18, 36, 52, 54, 88, 90, 118, 126, 158, 180, 206, 212, 216, 218, 250, 256, 262, 292, 298, 302, 306, 324, 326, 332, 338, 358, 368, 374, 410, 414, 448, 450, 508, 514, 530, 532, 540, 548, 556, 562, 576, 586, 594, 626, 632, 652, 682, 684, 692, 700, 710, 720

Offset

1,1

Comments

All but the first term are even.

Links

Table of n, a(n) for n=1..54.

Mathematica

Select[Range[750], PrimeOmega[# + 3] == PrimeOmega[# - 3] == 2 &] (* Vincenzo Librandi, Mar 28 2015 *)
SequencePosition[Table[If[PrimeOmega[n]==2, 1, 0], {n, 800}], {1, _, _, _, _, _, 1}][[All, 1]]+3 (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 21 2017 *)

Prog

(PARI) lista(nn, m=3) = {for (n=m+1, nn, if (bigomega(n-m)==2 && bigomega(n+m)==2, print1(n, ", ")); ); }
(Magma) IsSemiprime:=func< n | &+[k[2]: k in Factorization(n)] eq 2 >; [ n: n in [4..750] | IsSemiprime(n+3) and IsSemiprime(n-3) ]; // Vincenzo Librandi, Mar 28 2015

Crossrefs

Cf. A001358 (semiprimes).

Cf. A124936 (n-1 and n+1), A105571 (n-2 and n+2).

Cf. A256382 (n-4 and n+4), A256383 (n-5 and n+5).

Sequence in context: A030714 A190036 A061141 * A272975 A190495 A271988

Adjacent sequences: A256378 A256379 A256380 * A256382 A256383 A256384

Keyword

nonn

Author

Michel Marcus, Mar 27 2015

Status

approved