A189657 Start with n, apply k->2k+1 until a semiprime is reached; sequence gives the semiprimes.

15, 95, 15, 9, 95, 55, 15, 35, 39, 21, 95, 25, 55, 119, 511, 33, 35, 303, 39, 335, 87, 91, 95, 49, 51, 215, 55, 57, 119, 123, 511, 65, 543, 69, 143, 295, 303, 77, 159, 327, 335, 85, 87, 5759, 91, 93, 95, 391, 799, 203, 415, 54271, 215, 219, 111, 3647, 115

Offset

0,1

Comments

This is to semiprimes A001358 as A051919 is to primes A000040. Is this sequence defined for all n?

Links

Table of n, a(n) for n=0..56.

Example

a(0) = 15 in 4 steps because 2*(2*(2*((2*0)+1)+1)+1)+1 = 15 = 3*5 is semiprime.
a(1) = 15 in 3 steps because 2*(2*((2*1) + 1)+1)+1 = 15 = 3*5
a(2) = 95 in 5 steps because 2*(2*(2*(2*(2*2 + 1)+1)+1)+1)+1 = 95 = 5*19.

Mathematica

semiPrimeQ[n_] := Total[FactorInteger[n]][[2]]==2; Table[k = n; While[k = 2 k + 1; ! semiPrimeQ[k]]; k, {n, 100}] (* T. D. Noe, Apr 29 2011 *)

Crossrefs

Cf. A001358, A051919.

Sequence in context: A126483 A226766 A114240 * A190274 A374808 A052459

Adjacent sequences: A189654 A189655 A189656 * A189658 A189659 A189660

Keyword

nonn,easy

Author

Jonathan Vos Post, Apr 25 2011

Extensions

Extended by T. D. Noe, Apr 29 2011

Status

approved