

A256381


Numbers n such that n3 and n+3 are semiprimes.


4



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
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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(nm)==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(n3) ]; // Vincenzo Librandi, Mar 28 2015


CROSSREFS

Cf. A001358 (semiprimes).
Cf. A124936 (n1 and n+1), A105571 (n2 and n+2).
Cf. A256382 (n4 and n+4), A256383 (n5 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



