A103393 Semiprimes in A103373.

4, 9, 15, 21, 33, 38, 58, 65, 86, 106, 121, 249, 895, 989, 1199, 2059, 3073, 9206, 12302, 19766, 33238, 63109, 197459, 252982, 283942, 477931, 691357, 4598261, 8671301, 9819097, 11176233, 51113687, 205150267, 232797043, 496450043, 562358905

Offset

1,1

Comments

Intersection of A103373 with A001358.

Links

Table of n, a(n) for n=1..36.

Example

63109 is an element of this sequence because A103373(91) = 63109 and 63109 is semiprime because 63109 = 223 * 283 where both 223 and 283 are primes.

Mathematica

SemiprimeQ[n_] := Plus @@ FactorInteger[n][[All, 2]] == 2; k = 5; Do[a[n] = 1, {n, k + 1}]; a[n_] := a[n] = a[n - k] + a[n - k - 1]; Union[Select[Array[a, 170], SemiprimeQ]]
Select[LinearRecurrence[{0, 0, 0, 0, 1, 1}, {1, 1, 1, 1, 1, 1}, 200], PrimeOmega[ #] == 2&]//Union (* Harvey P. Dale, Jul 20 2019 *)

Crossrefs

Cf. A001358, A103373, A103383, A103392-A103401.

Sequence in context: A103397 A103394 A103395 * A103392 A073046 A380012

Adjacent sequences: A103390 A103391 A103392 * A103394 A103395 A103396

Keyword

nonn,easy

Author

Jonathan Vos Post, Feb 03 2005

Extensions

Edited and extended by Ray Chandler and Robert G. Wilson v, Feb 06 2005

Status

approved