A117916 a(1) = 4; a(n) is smallest semiprime > 3*a(n-1).

4, 14, 46, 141, 427, 1282, 3849, 11553, 34663, 104001, 312005, 936017, 2808053, 8424161, 25272487, 75817463, 227452391, 682357183, 2047071551, 6141214658, 18423643982, 55270931959, 165812795887, 497438387665, 1492315162999

Offset

1,1

Comments

Semiprime analog of A076656 a(1) = 2; a(n) is smallest prime > 3*a(n-1). a(n)-a(n-1) is often 1, never more than 16 through n = 28, then jumps to 32 for n = 29; and I wonder what its value or mean value is asymptotically as a function of n.

Links

Table of n, a(n) for n=1..25.

Mathematica

nxt[n_]:=Module[{c=3n, k=1}, While[PrimeOmega[c+k]!=2, k++]; c+k]; NestList[ nxt, 1, 30] (* Harvey P. Dale, May 31 2012 *)

Crossrefs

Cf. A001358, A055496, A076656.

Sequence in context: A232060 A302238 A171851 * A054449 A029868 A030267

Adjacent sequences: A117913 A117914 A117915 * A117917 A117918 A117919

Keyword

easy,nonn,less

Author

Jonathan Vos Post, May 04 2006

Status

approved