A256382 Numbers n such that n-4 and n+4 are semiprimes.

10, 18, 29, 30, 42, 53, 61, 73, 78, 81, 89, 90, 91, 115, 119, 125, 137, 138, 162, 165, 173, 181, 198, 205, 209, 210, 213, 217, 222, 258, 263, 291, 295, 299, 305, 323, 325, 330, 331, 390, 399, 407, 411, 441, 449, 450, 462, 477, 485, 489, 493, 497, 501, 515, 523

Offset

1,1

Comments

A117328 is the subsequence of primes.

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Mathematica

Select[Range[600], PrimeOmega[# + 4] == PrimeOmega[# - 4] == 2 &] (* Vincenzo Librandi, Mar 29 2015 *)
Flatten[Position[Partition[Table[If[PrimeOmega[n]==2, 1, 0], {n, 600}], 9, 1], _?(#[[1]]==#[[9]]==1&), {1}, Heads->False]]+4 (* Harvey P. Dale, Mar 29 2015 *)

Prog

(PARI) lista(nn, m=4) = {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 [6..750] | IsSemiprime(n+4) and IsSemiprime(n-4) ]; // Vincenzo Librandi, Mar 29 2015

Crossrefs

Cf. A001358 (semiprimes).

Cf. A117328 (with primes rather than semiprimes).

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

Cf. A256381 (n-3 and n+3), A256383 (n-5 and n+5).

Sequence in context: A244573 A230356 A100992 * A167607 A162828 A190038

Adjacent sequences: A256379 A256380 A256381 * A256383 A256384 A256385

Keyword

nonn

Author

Michel Marcus, Mar 27 2015

Status

approved